At most schools (all three I've attended, anyway), there usually is a company that comes through for a week sometime in the first few weeks of class, sets up in the student center, and sells a very large selection of posters. It may just be worth waiting until they come by.
The other thing to watch for is student art sales at the Fine Arts department of your school. I've bought some amazing work from these, and getting a large oil on canvas is cheaper than you'll find anywhere else. There's something really satisfying about a tangible piece of real art (versus a poster), so if you find something you like, I'd encourage you to pick it up.