BTW: if you're going to try skiboarding , you might want to think about a helmet, too Have fun!
To just get clothing and not rent equipment:
Sports Authority on Rockville Pike usually carries Columbia clothing and is probably your best bet for saving money on clothing.
In my experience, Ski Chalet and Ski Center generally carry higher end clothing.
Never been there, but you might also try Galyan's off of 270 near the now defunct Sportland.