Amazon, chegg, campusbooks, and textbookrentals are also great.
I would highly recommend renting from Amazon instead of buying! Renting a book (new or old) could be less than $30. While you don't get any money back when you return it, it's beneficial because often new versions of textbooks come out anyway and the book you try to sell back would be worth almost nothing. When you rent, you can keep the book for the whole semester with the option to extend, and Amazon also sends you a return shipping label for free when you are ready to send it back. This saved me hundreds of dollars in school.
I've bought textbooks, rented them and downloaded them onto my Kindle. I find that, for me, I do better with a physical text book. When I rent, I use a ton of sticky notes (since I can't highlight or write in the book). This approach has worked well.
There are many online providers of textbooks that will let you buy at a discount, so the internet may be your friend when dealing with texts. But I would highly recommend renting. Just remember to send them back in time, or incur a further charge!
I would recommend Chegg.com.
You can rent or buy. You can highlight in and write in rented books (to an extent). You can also sell any textbooks you may have bought elsewhere.
Chegg also has some unique features such as study and homework help!
Check it out!