Maryland Crab in DC?
I can't suggest a place to get Maryland crab in Maryland, let alone DC. The local crab industry is so devastated, much of the crab comes from the southern U.S., even around here where the conventional wisdom is that we must be loaded with it, like we were 30 years ago.
If you mean whole, steamed crabs, you're really talking about blue crabs and then it doesn't matter whether they come from Maryland or the more southern states. Same animal. Really. No kidding.
And the best place to get them for eating in near downtown DC is across the river at the Quarterdeck in Arlington. They claim that they're all local, but who really knows? http://www.qdrest.com
If you're looking for crabs to go, check out the markets at the waterfront on Maine Avenue.
And if you're thinking about crab dishes such as crabcakes, you'll most likely wind up with Asian swimming crab, unless the menu or waiter specifies that it's blue crab.