Another thing you can try, which is probably not entirely legal (but it's college, and textbooks are robbery in and of themselves), is to obtain Adobe Acrobat, get the nook software (which is free), and then get trials of the ebooks you need. Read them in the nook software, taking a screenshot and saving each page as an image (you can use scripts for this) and then using Acrobat to make a .pdf from the images. You can then proceed to scan the images for text so they are searchable as text. It's a time consuming process, but it will save a good bit of money.
Amazon I've found usually has the best prices as a lot of textbook sites appear on there through the community sellers. There's also a good number of college students who sell their books right on there so you avoid the store markup