I live here, and I can't think of anyplace other than the big gift shop on the lowest level of the Smithsonian American History Museum. If you didn't see them there, then your best option might be to look online. There is an online version of the Smithsonian store, although they don't have anything not found in the shops.
DC is kind of an odd town. Tourism is a big part of the area, but a lot of it is publicly funded, so they don't do the same level of merchandising you see in other touristy places. There are some great gift shops! They just don't always have new merchandise for current or temporary exhibits. My favorite gift shops for finding new things are the shops for the National Gallery of Art and the Spy Museum.