Try H street in DC
There are a couple of markets in Rockville:http://www.yelp.com/biz/maxim-gourmet-or��http://www.yelp.com/biz/kam-sam-supermar��Ignore the person who said H St in DC. That was the old Chinatown in DC, and there are still a few good Chinese restaurants, but the Chinese community has almost completely moved out, and there are no Chinese food markets left there.
Rockville ...do you know where Wooton High School is? the whole school is filled with chinese people so I would say that is where the community is.