Try Nature by Design in Alexandria. They're tucked away off Route 1 on Calvert Avenue, but it's well worth hunting for -- a treasure trove of natives!!

Here is a link that might be useful: Nature By Design's webpage

My favorite plant source in the DC area is the sales area at Green Springs gardens in Alexandria. They always have tons of member-propagated things, many unusual and hard to find, and the prices are great. If you become a member of the gardens ("FROGS"), you get 10% off of anything you purchase. Their spring sale a couple of Saturdays, with vendors from all over the east coast, was one of the most amazing such events I've ever attended.