I've bought vinyl at CD Cellar in Arlington, though the Falls Church location also has vinyl. Decent selection, prices & generally condition of vinyl is pretty good.
If you're looking for really cheap vinyl, and a decent selection of classical, Second Story Books in Rockville now has a vinyl section. $50 to fill a box (100+ records). Doesn't get much cheaper than that. Not so great for non-classical - lots of John Denver, 70-80's rejects, etc.