Some Ebay seller do sell the pdf version of books that can be purchased online. But, because its illegal, its hard to find. I had found one a couple of years ago and had paid $5 for a $150 book.

This is what i used to do when i was in college. I used to email professors saying that i already had a book for the course but they are an edition older. Almost all professor 99% used to reply me saying its ok and also asked me to consult them if i have any problems finding the materials. Then, i would go on ebay and purchase older edition books. I never had any issues and saved hundreds of dollars.

Try looking for some international ed. though Ebay. They are cheap and contain all the materials as the original.

Also, you can rent the book and send it back when those are due. Go to chegg.com or other websites for rental info. They are cheaper to rent and you do not even have to worry about getting rid of them once your done.