Are you looking for belly lox or nova? If belly lox, the only place I can find it in DC is Morty's, and it's fine. Not as thin as I might like or as great in terms of texture as what I grew up on, but it will do. If you're fine with Nova, you can find that at Balducci's.
If you're serious about getting really great smoked fish, though, consider ordering from Russ and Daughters or Barney Greengrass in New York. They send it packed in dry ice. It's expensive but worth it for a special occasion. You can also get sable, whitefish, etc.
http://www.russanddaughters.com/
http://www.barneygreengrass.com/welcome.php
Another option is Wegman's; my parents live outside of Baltimore and they get really great lox at Wegman's. But you'll have to schlep pretty far out to find a Wegman's.