A reader (!) wrote in asking for recommendations about back issue stores in the area. Here's an edited version of my replay and suggestions are welcome -
Very few back issue stores left - there's one in Vienna, VA - which is not the Big Planet Comics store there. According to Randy T in the comments, it's EG Comics, which is not on this this Google search, but is on Maple Ave in one of the strip malls. Aside from that - Barbarian Books in Wheaton - call first as they have odd hours and are closed on Monday I think, although they are close to a Metro. Maybe the closest.
Obviously I'm a fan of Big Planet - I go to the Bethesda store - but they're all good and they all have almost no back issues now (except for College Park - see Joel Pollack's comment below). Another good store is Fantom Comics in Tenleytown and Union Station, both locations right outside the metro. No back issues, or not many, though.
Most back issues turn up at the two local cons - the Capital Associates one is bigger and had a good selection of back issues when I was at it last Sunday.