Yes, universities have bookstores, but the campus bookstore is often the LEAST economical way to buy books. I frequented half.com as an undergrad, not only for traditional textbooks but also like paperback novels/plays for literature classes (which can sometimes be found basically for pocket change). Like Owlie said, sometimes new books are cheaper online (Amazon etc) than used ones are from the campus store, which is somewhat counterintuitive. But with Amazon's free budget shipping, it can be a way to save money.

This semester, I'm trying a textbook rental from Barnes and Noble for the first time. I knew that it was a textbook that I was not going to want to keep for reference, I wasn't finding used copies anywhere, and it was published 3 years ago so the chances of a new edition coming out and cutting the resale value is only going to get worse from here, so I decided to do a rental. The book retailed for $112 new and I'm renting it for a semester for $63 or something like that.

Sometimes there will be an "unsanctioned" independently-owned bookstore off campus that competes with the campus bookstore. Both universities I've attended have had something like this nearby, and they would distribute coupons for $15 off a $100 purchase and the like, so that's another option to look into to save a few dollars.