An easy way to get some is to get paid cash for your used books. A cool site that you may not be aware of that pays cash via Paypal for your used books is . They buy all types of books, not just textbooks (although they do buy back used textbooks too).
Here’s how it works: You go to the , input the ISBN from your used books (found in the first few pages of your books), and then see how much they will pay for your books. Then you print out a prepaid mailing label and mail your books to them. They even pay for all shipping costs! You will get paid for your books via Paypal or check, your preference. How easy is that? And there are no out of pocket costs to you. We have been paid by Paypal many times from this site, so we can confirm it is legitimate!
So give it a try, you get some extra cash for yourself too, AND clean out all the clutter from your bookshelves: