Every students needs books, and a budget student needs budget bookstores. Here are some quite independent, cozy and popular ones which sell books new and old, trade books, cds and even comic books!
Pretty Price Antikvariat
Used books and you can trade some.
Also, a very popular store named Tronsmo (not necessarily budget), carry a lot of books you might have some trouble getting elsewhere. Try them for a good experience at Kristian Augusts gate 19:
Also, Fleamarkets are quite common, so just keep your eyes open for banners in the street! its called LOPPEMARKED (flea = loppe, marked =pretty obvious). They are worth a try, and some people have found great deals in them. Every Sunday there is one in the Birkelunden park in Grunerløkka.