I've been asked to shoot some fantasy and historically themed stock photography for a friend's Graphic Design class.

Rather than run out and buy costumes from the store, where could I find pieces for costumes on the cheap?

If you have a local theater company, maybe more than one, call them (or go by the theater, better yet), and find out if you would be allowed to use some of their costumes or props.

I'd recommend explaining the entire set-up, idea, time of use, etc. to whomever it is that's in charge of the props/costumes...and even say something like, "I'm willing to work with your schedule"...

Be nice, you might get what you need for free!