Welcome!
It's been awhile since I lived in DC but don't remember any shops downtown... best to get on the metro and head out to Bethesda or Rockville if ya wanna buy a kit... There's one in Federal Plaze in Rockville I know of...
I looked it up and it's at:
Hobby Works
Federal Plaza
12274-G Rockville Pike
Rockville, MD 20852
Phone: (301) 468-6330
Take the Red line metro train to Twinbrook station and it's less than a 2 minute walk to the strip mall...
----edit----
just thought I would explain that better... the metro is the train system which you probably got, it will cost about $2.20 at the most to get there... (it was 1.80 when I was there, just allowing for inflation)
Just find a metro station and tell them you need to get to Rockville (Twinbrook platform) and they will put you on the right track. It's only about 20 min or so from Downtown by train. The twinbrook station is almost right at the store itself...
(in case this is your first trip)
For anything else you want to do in D.C. when you are there, the Metro is your best bet.. renting a car and driving is not worth it unless you want to go into Virginia or Md (outside of the metro system), the traffic is just too bad.. it's cheap, clean, there are always Metro police on them so it's safe... The people in the kiosks at each station will help you with a guide for which train to ride to which stop and even if it's off the rail line a little bit, tell you which bus to tranfer to and give you a free ticket for the bus! I lived there for 6 years and still took the metro any chance I got...
Enjoy your trip!