Where can I buy digital textbooks and not rent them?

I want to buy my textbooks in electronic format but I do not want to rent them. I want to save them for reference later on like I do with my math books.Where can I buy digital textbooks and not rent them?
Below are places where you can buy digital textbooks



http://www.zinio.com/

http://www.ecampus.com

https://ebooks.primisonline.com/eBooksto鈥?/a> (McGraw Hill digital textbooks)

http://www.coursesmart.com/Where can I buy digital textbooks and not rent them?
I use UsedTextbookPrices.com. Its a free search website that will find the lowest price of any book for you on like 30 book websites.