eBay
Amazon

Seems to be a lot of good online options.
Related threads to check out:
– one
– two
Another cheap option might be to see
if they have textbook swaps on campus.

But you know what’s better than cheap? Free.
Consider taking advantage of the reserve copies in the library.
I used to do this for the textbooks that I didn’t want to keep after the class was over.
Saved me ridiculous amounts of money. And actually got my butt into the library to study.

I try to find older editions and see if the text is the same. That saves a lot of money.

http://www.cheapbooks.com/ is also pretty good.

(And there’s always ebay and amazon as aforementioned).