There are a few out there, but the one I found recently that has usually been great is chegg.com. You sometimes have to be careful, as it's a resale textbook site, and if you need access keys for online tools linked to a textbook, you can't get them (even if it's advertised!).
Between chegg and
amazon, I've been able to cut my textbook costs down pretty substantially. Renting is often a great way to go if you don't need the book for future classes or as a reference book. If you do rent, check the price different by state for Amazon (they have a pull-down). As an example, the cost to rent and ship to MN was $120 for one of my books and only $40 to ship to TX. I have relatives in Texas, so I'll just have them ship the book to me (nothing in the rental agreement and terms shows this isn't ok, so why not save $80).
Lastly, if you're taking an online class, look at the cost to just get access to the publisher's website. This may come with digital access to a textbook and allows you to use a search feature to find things in the text quickly. You can print off a certain number of pages of the textbook as well (they don't let you print the entire book though). The last option is an e-book throgh Barnes & Noble or Amazon's Kindle.
Hopefully this helps. I've found that Amazon and Chegg were the best price option for me over the last three semesters. Very rarely did my college bookstore have competative pricing, so I feel your pain!