http://www.bookfinder.com is the resource I sent my students to - it looks at all the web resources out there and ranks them by price.
Some college textbooks don't read well via e-book because you can't "write" in the e-books as well. There are hopes that the next generation of e-books will have tools that will let you underline, highlight, and write notes in the margins of your e-books. If you're a highlighter, just keep in mind this limitation.
Whatever you do, don't try to save money by getting a different edition (the 2nd edition instead of the 3rd, or an international edition instead of the US edition) - your instructor will refer to page numbers and may assign problem sets from your book, and if you have the wrong version it causes headaches for everyone.