Funny you should ask. It's the time of year the new seed catalogs are mailed out. I tend to get seeds from everywhere. I use heirloom and hybrid seeds. Both have their merits and neither is always superior to the other. It depends on the variety of heirloom or hybrid you are talking about. In a nutshell this is my opinion of how they stack up:

Heirlooms-
Positives:

1. Often, but not always, they come in a wider variety of colors, shapes, and flavor profiles than hybrids.
2. You can save your own seeds and trade/share with others.
3. The warm fuzzy you get from growing and eating the same things previous generations have enjoyed and made an effort to preserve.

4. There are also a lot of newer open pollinated varieties that are not heirlooms, but may become the heirlooms of tomorrow. Also you are helping to preserve genetic diversity.

Negatives:

1. Disease resistance and vigor is more variable than hybrids. There are some very healthy heirlooms, but finding them is hit or miss.

Hybrids:

Positives-

1. They have been scientifically bred for specific traits of disease resistance or quality. You can know ahead of time what disease your seed will resist.
2. They may have "hybrid vigor." The variety of genes they get from both parents may make them a little healthier.

Negatives:
1. You have to buy new seeds every year.
2. Taste is sometimes more variable than heirlooms. Many hybrids have been bred for traits other than taste. There are many hybrids that taste as good as many heirlooms however.

Anyway I use both :)

Big Box Stores (HD, Lowes, Kmart)
Pinetree Garden Seeds
Baker Creek
Sandhill Preservation
Sampleseedshop
Renee's Garden Seeds
Tatiana's TOMATObase
Johnny's Selected Seeds (there is a free shipping code now/google it)
That's some of them.

Thanks for the reply.

Florida based businesses ----
From their website:
http://www.tomatogrowers.com/
Tomato Growers Supply Company has a free catalog of tomato seeds, pepper seeds, and eggplant seeds. More than 500 varieties of tomatoes and peppers, including huge selections of both hybrid tomato seeds and heirloom tomato seeds, hot chiles, sweet peppers, tomatillos and eggplants, plus garden supplies, garden books, and posters.

Osceola Farm & Ranch Supply East LL c
(407) 892-4040
215 13th St, Saint Cloud, FL 34769
Ask for their seeds. They will also order anything special you want.
Feed stores are a good place to check for seeds and other veggies supplies.

Places I like to buy from:

Pinetree Garden Seeds
Southern Exposure Seed Exchange
Tomato Growers Supply Co.

& another 1 here in FL I (re)discovered recently - they sell milky spore treatment by the ounce, along w/ unusual tropical, heirloom & veggie seeds Eden Organic Seeds

Very good information here for those of us venturing into sustainable gardening. Thank you all for these responses.

Oh, and Seed Savers Exchange. I just got their new catalog today.

the cheapest place I've found to get seeds is the farmer's market. Go buy a great, locally-grown, organic cantelope and the seeds from that fruit will probably do well in your garden. If you think about it, these are fruits from farmers who run businesses, so they spend a lot of time trying to figure out what works best for your local conditions. And they are organic, so they're looking for disease and pest resistance in your area. Even if you're not going to do organic, the varieties that they plant will require less pest-care than something you find online that is geared toward anyone in the country and any pest in the country. Our bugs are different from the rest of the country.

It's not fool-proof, but it's given me some good results. And I've talked to the folks at the farmer's markets about it and gotten good advice as a bonus - they're generally pretty happy to talk about their farming because they're proud of it. They can tell you about how they grow the tomatoes/peppers/whatever and what works for them in terms of soil amendments, etc., plus what pests they face.
It was a tomato grower who first told me that coffee shops, including Starbucks, will give you their coffee grounds for free. Just knowing that and using my own coffee grounds saved a gardenia I had given up on. So they're a neat wealth of information.
And that's the way backyard gardening should be done, right? We all share plants, information, tips, seeds, etc.

That's a good tip about getting seeds from the produce at the farmers market, but you should probably ask the seller about it first as the previous poster suggested. The farmer may be growing hybrid varieties or even if it is open pollinated, a given farmer may have several kinds of melons, squash, etc... growing next to each other and the seeds may have been cross pollinated. Don't save the seeds from hybrids unless you are willing to get a surprise.

This is how I have started getting my herbs. I go to the Marion Market which has a produce section for local farmers. When I purchase my herbs or veggies, I always get one or two 'extra' to bring home and plant. I guess in a sense it kind of shafts the growers as I no longer need to purchase basil, oregano, parsley or spring onions from them because I have a small, sustainable crop going in my containers and yard. But, it saves money for me in the long run, which I need to do, and keeps things right there on-hand for when I need it.