I'm kind of new and this post is a little old to be replying to I guess, but I found it in a search about bindings that work with the 3D system, go figure! I'm going to have to deal with the issue pretty soon since my boyfriend is giving me his old board... haha
As for shops around DC, it's not just REI and Sports Authority. Not sure exactly where around DC you live, but if you have access to a car you could try Willi's in Fairfax (they're moving in a few weeks to a new location in the same city, so they will be closed for a few days around Valentine's Day), Alpine Ski Shop out in Sterling (been around forever, I bought pants from them like 10 years ago for my first ski trip). There's also Fairfax Surf Shop in Fairfax City that also should be able to help you out and I believe
they are metro/bus accessible being smack dab in the middle of the city. Ski Chalet has a few locations around here, one in Arlington, but not particularly close to a metrorail stop, might also be bus accessible. Not sure how helpful they are, though. I'd probably try one of the other places first.
Plenty of shops you could get some real help at. My best advice as a relative noob would just echo what you found via search and try one of the places I listed. Good luck and if you already found a plate that works, now you know some awesome places to check out in the future.