Copy shop's probably best if you want low #s of sheets, and there must be one in walking distance of campus, right? Otherwise, do you have the equivalent of OfficeMax or OfficeDepot near you?
An office supply company will have the best selection. 13x19 is a bit harder to find, whereas 11x17 is fairly easy. But an office supply company is going to have bad prices. Not as bad as a college bookstore, but still...
I'm new to this forum but happened upon this question and had an answer - www.atlex.com - I've bought 11x17 and 13x19 paper from them a number of times (along with toner cartridges). They have good prices compared to retail and speedy shipping. Check them out!