Well, there's a list of stores here
https://wiki.mobileread.com/wiki/E-book_stores, or for public domain books, check the downloads here on MobileRead - see the E-Books link up near the top of the page!
Personally, I read a lot of SF, so use
http://www.webscription.net/, where I'll use the LRF format, else from other sites such as FictionWise or BooksOnBoard, I'll download the MS LIT format books and use "Convert GUI" & Calibre's "lit2lrf" utility to convert to LRF.
No suggestions for textbooks, I'm afraid...