I thought I'd answer my own question in case anyone else is interested -- a neighbor emailed me the following link to the Audobon Society of Central MD's native plant sale, to take place April 29th.

Here is a link that might be useful: Audobn Society Native Plant Sale

Irvine Nature Center started selling trees & shrubs April 1.
The sale runs through May 20. They are located in Baltimore County.

Here is a link that might be useful: Irvi ne Nature Center

FYI for next year, The National Arboretum holds a native plant sale at the end of March every year in conjuction with their Lahr Symposium.

One of the vendors was Doyle Farm nursery. They are located in Pennsylvannia just over the Maryland border, above Jarretsville. Their website lists native plant sales in the area they will be attending. (It's on their "tours and open house" link, http://www.doylefarm.com/events.html) One sale is in Fairfax VA, Saturday April 29. Others are being held at Ladew Topiary Gardens, The Baltimore Herb Festiva, etc.,

While they sell by mailorder its nice to see the plants you're buying. The plants I bought from them at The Arboretum, White Turtlehead (Chelone Glabra) and Nodding Ladies Tresses (Spiranthes Cernua) are growing really well.

If you find any other sales in the Maryland area please post them, I'd love to know about them. :)
-Helen

Saturday, March 26th is the 25th year that the Lahr Symposium will be held. This year it will be at the BELTSVILLE RESEARCH CTR LOCATION due to construction at the Nat'l Arboretum.
By now the seminar part is probably full however the plant sales are free and open to the public. Here you find the things that you never find at the big boxes.

Here is a link that might be useful: Lahr Symposium

Saturday March 23 the National Arboretum will hold its 27th Annual Lahr Symposium & Native Plant Sale. Just like in the past, there will be great plants for sale direct from the growers who propagated and grew them. Sale is seperate from the presentations and is open to the public.

Here is a link that might be useful: Lahr Symposium

Try the Adkins Arboretum on the Eastern Shore

Saturday March 28 the National Arboretum will hold its 29th Annual Lahr Symposium & Native Plant Sale. Although at this point the seminar is full there will be several top quality vendors in the parking area selling plants. What a great chance to buy plants directly from the grower and the plant sale is open to the general public.Lahr Native Plant Symposium

2016 Lahr Symposium is scheduled for April 2. By now the seminar is sold out. The plant sale will be held outside and is free and open to the public. US National Arboretum is the location with a lot of really great plants available.

Its hard to believe the 31st annual Lahr symposium will be held next Saturday, March 25th at the National Arboretum. The plant sale, which is held outside is free and open to the public. Don't miss this chance to buy native plants directly from the people who propagated and grew them.

Other options:

Herring Run Nursery in Baltimore now has hours during the week, in addition to the weekends (it's run by volunteers); Irvine nature center has a sale in August. Schott Nursery in Taneytown is a wholesale nursery which concentrates on natives (trees and shrubs) and has retail hours Saturdays 8-12 (email or call to pick up what you want, they have an availability list on their website--fantastic prices). Ladew Topiary Gardens has a native plant sale coming up soon too. Maryland Sheep and Wool has native plant vendors by the front entrance and they'll hold your items while you enjoy the festival (I think Putnam is one of the vendors). That's all I can think of off the top of my head.

Hi Anna, please tell us more about the Ladew Native Plant Sale, don't see anything on their website.

Hi Sam, general admission on the bottom right:

http://www.ladewgardens.com/EVENT-CALENDAR/Garden-Festival

I have been to the Audubon society sale mentioned above by another poster but once it became very popular I stopped going. You have to get there EARLY and they only have a few of the more "desirable" plants. It's a madhouse.

I make a point of going to the Irvine Nature center as they feature many different vendors and the prices are good, quality is good.

Just went to Schott this AM and loved it. Great staff, prices are insane, great quality. Purchased a Calycanthus fluoridus for a friend and it was such a lovely specimen.