If you want to buy some fireworks, get yourself to 14th and Newton, where there's a legal stand. They don't sell any that shoot into the air and explode (or the tanks that drive around and shoot sparks, which I like) but do sell lots that spray colored sparks in the air and make noise, plus fun, tamer stuff like sparklers.
DCRA usually posts a list of all the legal fireworks stands in the city, and when they do, I'll post that too.
In the meantime, have fun, and be safe. I already heard some folks shooting off fireworks this past weekend.