I buy from any place that has plants I want. Most times the local nurseries are about the same prices, with better selection. Occasionally the big box stores get unusual varieties, I guess that depends on the buyer. Not all Lowe's, for example, carry the same plants. The local big box stores usually carry roses, coneflowers, most everything, at least one cultivar.

I believe the nureries take better care of their plants, water, fertilizer, etc and the quality is usually better as they have an investment to take care of. If the plants die, they take the loss. Big box stores often don't care if their plants are taken care of, a lot of them (HD for one) are pay-by-scan and the stores don't actually pay for the plants until they are sold, so they have no investment and it isn't their loss if plants die. Best time to buy from them is first thing in the spring when they first get them in and haven't had time to mishandle them.

Besides, I really prefer to support the local nurseries when I can rather than some big corporation that has no investment or ties in the community.

Some local nurseries will sell bare root stock at a discount early in the spring, when they first come in, before they are potted up for the spring. That's when I usually buy mine, in March here. You would have to ask if you can do that.

Good luck with your project, I hope you will post pictures!

I shop at local nurseries and via mail-order for the items I can't get locally. I don't shop at the garden departments of the big box places and don't suggest them for anyone except for summer annuals.

Your local nurseries in my experience have wider selections, better quality stock, more knowledgeable staff, and are members of your community. Your money spent there stays in your community.

If you have a large project, talk with your local nurseries. They may offer some special deals, maybe not. But they are more willing to do a one off type of arrangement.

Since you are in a northern climate, late summer - fall is the absolutely best prices on plants. People are trying to move stock so they don't have to take a loss or try to overwinter it. I've gotten potted roses for as low as $2 a piece, perennials in gallon pots for $1, and more. Some don't make it and some look rough at the time but come next spring, they look just fine. You have to sometimes see the diamond in the rough but the prices are more than right to take the chance.

The local nursery that sells bareroot stock sells their warehouse bareroot stock for 50% off come June. It's a so-so time to plant trees so I just pot up the bareroots I buy at that time, baby them until fall and plant then. I have a fairly good success rate with that.

FataMorgana

If willing to wait a bit you can grow some plants like coneflowers from seed to flower in two years, sometimes one year if they are started indoors early. Also plugs can be a cheap alternative to potted plants for large projects.

The big box stores may have some good clearance plants, but I wouldn't expect a huge selection. Sometimes you can find bargains at nurseries at the end of the season. 3 years ago over Veteran's day weekend, I purchased 55 perennials and grasses (mostly native) from the local nursery for $3-5 each. These were generally really nice cultivars like Baptisia 'Twilight Prairie blues' and Sedum 'Matrona'. Some mail-order companies sell flats of small plants for a good price.

If you have a little patience, starting plants from seed is a great way to grow a huge variety of perennials really cheap. Winter-sowing works great. The small seedlings can be planted or potted in small pots to grow on the first year. I try to get most in the ground by winter, but always have to overwinter some and plants in pots do generally need to be protected over the winter. A few perennials will flower a bit the first year from seed, but most will take 2-3 years. It really doesn't take all that long to fill up a large garden, especially if you fill in with annuals while the perennials are small.

Plant swaps are also a great way to get plants (and they're free!).

I have found that in this area most of the nurseries are wholesale only. The few that actually do sell to individuals are very good to deal with. Most of them are related to the other nursery owners and can get something they don't have in stock.

They are also very reasonable on their delivery charges.

I never buy from big box stores. They all seem to die when I do. I think they must force feed drugs to them , I mean fertilizer, and then when they hit my soil , they just shrivel up and die jonseing for a fix.. Whereas when I go to my three favorite nurseries that carry natives and plants that like my climate, they live quite well. I grow a lot from seed now also. I have 50 natives that I collected from my trip to New Mexico.

For good price in large quantities the mail order places tend to be competitive even when you add in the cost of shipping. If you are lucky, there will also be a specialist native nursery within a distance you can travel to. It's a mixed bag when it comes to price at big box stores versus local nurseries in my experience. Sometimes I get better prices at one and sometimes the other. Nurseries do tend to take better care of their plants. From the tags I've seen on pots at big box stores it looks like they buy their plants in very large quantities from mass producers, consignment growers, etc. and not from small or local or retail operations. If you are willing to nurse a poorly cared for plant back to health you can often get low prices near the end of the selling season at big box stores and sometimes even at supermarkets. Around here there tends not to be a huge variety of natives available at big box stores or local nurseries. Once in a while they will have something not so common however. BTW, I have seen black eyed susans and purple coneflowers for sale at most nurseries and big box stores around here for years now so there's two natives at least that tend to be readily available -- at least around here. Also, BTW, if your definition of native includes Cosmos (they're from this continent anyway), they are pretty easy from seed. I literally just tossed some Cosmos seed on the ground and they grew into big healthy flowers and reseeded and did it all over again every year for the last few years. I also got lucky with Partridge Pea seed but they eventually went away after a few years with maybe a few still popping up. My transplanted Bee Balms have multiplied without any help so that's something as well (in some gardens they do that too much).

I second what terrene said about winter-sowing. It's easy, cheap and abundantly rewarding. Winter-sown plants will often flower the first year, but most will take a year or two to fully mature and fill out. Don't overlook farmers markets, nature centers and organizations like Wild Ones which often host yearly or twice-a-year sales of native plants. I mostly winter-sow, but fill-in with purchased plants to expand diversity.
I was a garden manager at HD. There's not much variety there and the plants are sometimes not looked after carefully by employees. Many are grown in another region of the country, so they may not be particularly hardy for your area. Ask where the plant originated from before purchasing from a nursery or garden center. Plants in full flower sell better, so vendors push the flowering for that reason alone, even though it may not be best for the health of the plant. It's best to research what you want, make a list and go from there. Look for plant vigor rather than flower power. A small plug will outpace and be healthier in the long run than a bigger plant in most cases. So don't pay for instant gratification. Nurseries often use the same vendors as big box stores so don't automatically assume they have better stock delivered to their stores.

Buying from a native plant nursery in your region is probably a smart start. If you can't find one, perhaps you can mail order seed or plants (often bare root) that are native to your region. The problem with mail order, as well much of the stock in local nurseries, is that even though they may sell plant species native to your area, the plants may have originated from areas with significantly different conditions, and not have the survival success you desire. Prairie plants started from seed is an economical way to go. Try a search engine and type in native plants, trees, etc, and perhaps your state or region to see if you have any options.

rickstang, if you're still around, I'd like to add a couple of points.

First, if you have particular plants in mind, you could talk to a local nursery and find out if they're likely to have them in stock, and when they'll have them in stock. If they won't have the variety you want, or won't have enough of them for you, they may be able to obtain them -- or grow them -- for you.

Depending what you're looking for, you may have to look at the optimum time. Not everything is available year-round, particularly at the big-box stores.

I rarely find anything that I would want to plant at a big box store.

Most of my natives come from plant rescue or mail order.

there are a couple specialty nurseries that have stuff that I would plant, but they are a far drive. I have pre-ordered and then picked them up when they were ready.

Arrowhead Alpine Nurseries is a great independant plantsman that can and will advise and assist if you go to his website get his phone # and call him in the evening.He is brilliant and will cut you a deal.