Rantings of a New Yorker

Try these places.  You can get most of your books just by checking around the Internet.

Ebray
Google Books
PaperBackSwap
Populair
The Frugal Panda’s List

You can also do a “filetype:pdf book title” on google or check for the international version of a book, which is usually a lot cheaper.  Many students also print or photocopy copies of textbooks.  Some professors even have 1-2 copies that they loan to hardship students.  They may have also placed copies in the library for students check out.  It doesn’t hurt to ask.  You can also see if your college or university has textbook scholarships.