There really isn't anything much for thrifts in the District. No idea why... Most of them are outside DC that are any good...
There are a couple of MCM stores in the District, but if you want anything from them, you better be a million dollar a year attorney, because they aren't cheap at all...
The guys that spartanmanor mentions are in an antique mall about a mile from my house.. Here's the address.. His friends do have some really nice stuff, and I often go look to see if they have anything I'd want...
The Antique Center
8101 Richmond Highway (Route 1), Alexandria 22309
About a half mile from there is a thrift store, that sometimes you MIGHT find something. about all I've gotten from it are a pair of Cornwalls, and a Kenwood tuner.. You have to be there to get anything as they put it out... Too many people that look for gear to flip in the area..
There is also a Goodwill on Rt. 1, but they seldom have any decent gear either.. I check it often, as do all the other flippers in the area..
All down Rt. 1 between Richmond and DC are thrifts and Antique stores. My wife and I hit them every now and then, and also very little there gear wise, but we do find other cool stuff we like... ... There are all kinds of other thrifts in the area, but you have to be there at the perfect time... Granted, there are a lot of people in the area that do give good stuff to the thrifts, but there are also a ton of flippers in the area always looking and checking trying to make a buck..
Good luck is about the best advice I can give you.
Pretty much all the gear I've gotten in the area either came from Craigslist, or from estate sales.. Pretty much nothing from thrifts, and we shop them a lot for other items we like..