if thriftbooks doesn’t have what you’re looking for, especially if you’re looking for it used/cheap, alternatives include betterworldbooks and discoverbooks.
Other tips for cheaper books is checking amazon and scrolling down to the “buy used” or “other sellers” section and then checking to see if those sellers have storerfonts off of amazon.
There is also a good chance that you might have a local, indie book store that likely also has a used book section! Indiebound might help you find those book stores!
Also, check out your local library, sometimes, they have a for sale section or might have certain times of the year where they do massive used book (dvd, cd, etc) sales to fundraise!
(It’s also worth checking directly from an author’s or publisher’s page and seeing if they have other places they sell their books. I know this is about cheaper/used books, since some of y’all aren’t built for piracy or the library, but also if you want to dodge supporting amazon AND want to more directly support artists you support, there’s a good chance they might have alternate ways to buy the books!)
