Neopol Smokery is great and has a wide selection. Main store in Baltimore, but with a stall at Union Market in NE DC, and also at some farmer's markets in DC and MD - see
http://neopolsmokery.com/the-shops/farmers-markets for locations.
Bullfrog Bagels (H St NE) cures their own lox and smokes their own whitefish; haven't tried the salmon but whitefish salad was very good.