Hi,
I went to Washington on holiday a few weeks ago. Stayed just off the Mall, by the Capitol Building, so quite close to the convention centre. There's not a large amount of shops there, it's mainly landmarks, sites, museums and attractions. I did go to Tyson's Corner shopping mall which is a Metro journey and bus ride away. That has an actual Lego brand store in it. Apart from that, I didn't see a TRU or Target anywhere. Not saying there weren't any, just that I never saw them.
Hope this helps.