Lets see, In bowie theres one that is actually a comercial breeder for angelfish, can't remember the name, but they advertise in the back of TFH. In Bladensburg there is one, or used to be. I havn't been to either one of these in years, so don't know where they are, just general areas.
The real place to go is the baltimore area. The Aquarium center in randallstown, exit 18b off the baltimore beltway, is absolutely awesome. There is also exotic aquatics in north baltimore, and house of tropicals off of route 2, just off the baltimore beltway. Bring a big cooler, lots of money. These places have an awesome selection of stuff you just don't see anywhere else in the baltimore, washington area.