When I tell people about how I get my books, I'm often met with blank stares and comments like "why couldn't you have told me about that three years ago!" Some of this is common sense and others just seem stupid, I'm sure. But if they help you save as much money as they've saved me, it will be worth the read.
The first thing I'd like to note is that you can almost always use older editions of textbooks, even if the instructor says not to. You might have to borrow a friend's book to see how they match up, but what usually happens is that publishers will add a little bit of new content and then mix the chapters up a bit. Maybe update some pictures. Nothing more. All of the learning is still there for the taking.
Every school has bulletin boards in them, and there are even places that are designated for book selling. A few minutes scouring these boards can save big money for students. And as was mentioned before, don't dismiss old editions. They will do.
I have bought the majority of my books from Amazon.com and Amazon.ca. They usually have the textbooks you need and they're usually about 30% cheaper than buying them from your school bookstore. You should order them early, though, just in case they take a while to ship. A trick to this is finding out the books you need early. I usually email a prof right after I register and ask which books are going to be used. They're more than happy to help.
Your school has a library that is stacked with books and journals. I have found that about 30% of the textbooks that I have used have also been available in the library. Sometimes it's the older edition, but as we have learned, that's ok most of the time. The only problem with this way of getting books is that you only have the book for a short time. While you can renew, if someone else puts it on hold, you will have to return it or pay the fine. Still, a good resource.
I have borrowed and bought a lot of textbooks from friends. If you find out who has taken the course before you, you can ask if the person has the book to lend you. Or maybe they want to sell it. This way, you not only get the book, but maybe you can get some good advice on the course. If you're really inspired, you can even set up an email list so all of your cohort can trade books back and forth. This option is especially applicable to social workers, now isn't it?
Well, that's about it. With these ways of getting books, I've saved more than 50% on what I was supposed to have paid for textbooks over my four years of studying. Oh, and always remember to sell or give away the books that you don't want to save for future reference, thus passing along the good karma and helping other save money, too.