Finding the perfect Halloween costume can be a nightmare. (ha). The Washingtonian just did a great article on places all over the city (even in my neck of the woods, Takoma Park) to find something. Check out the article because before you know it- you will be costume-less and sad.
http://www.washingtonian.com/blogarticles/artsfun/afterhours/5482.html
The article is also a super helpful reference for various vintage stores in the city so you better bookmark this one.