amazon.com! I paid $475 for books this semester on campus, then i found em on amazon for $205 shipped, and returned the ones i had. The trick: get an older edition of the textbook if you can. Two of my hundred dollar books i got old editions of for $4-5
I always bought from Amazon.com. While I was an undergrad I ran a scam on my parents to get more money. Buy the books from the campus store, get my parents to reimburse me, buy the books either new, used, or softcover (international editions = CHEAP!!!) from Amazon.com, and then return the books to the campus store when the replacements came in the mail. I would net about $400 per semester from my parents. That buys quite a bit of alcohol!