How about the library?
At my university the professors always put at least one copy of each of the required texts on hold at the library in a section where you could check the book out for like an hour at a time to do your readings.
But the library usually had several copies of all the texts so you could take your reading list and go and just check the books out and renew them throughout the semester.
Once I found this out I stopped buying books all together. I just wish I'd have done it for the first two years of school and saved all that money.
You could also try Paperbackswap.com