Try Amazon and eBuy. you will find cheap used textbook on there. Just use the ISBN to seach the textbook that you want.
I purchased all of my books directly from my University Bookstore and spent HUNDREDS of dollars I did not need to. As the years have gone by, I have found several reliable sources for textbooks.
First, I always look on Chegg. They have options to rent or buy books. In addition, you can sell textbooks when you are finished. Here is the general link: https://www.chegg.com/
Second, I look on Amazon. They also have options to rent or buy books. Here is the general link: https://www.amazon.com/
The easiest way to search books is by the ISBN number or by title and author.
Good luck and happy book searching!
Try used, rental, or ebooks for text required for your courses (locally or on-line). Watch the edition because sometimes it makes a difference.
Michael recommends the following next steps: