Lifeline 1: You probably don't need to buy books, even if they say required. Always always always check Libgen (link may change, if it doesn't work just google Libgen in various ways until it works), and check your library. See if your library has some time of "scan and deliver" service where they will scan book chapters for you if you just need a chapter, and email it to you via PDF.
Lifeline 2: If you do have to buy books, (and if you are like me and trying to avoid Amazon, for reasons such as those outlined in this Vox article), try searching on book finder websites like addall books, or ebay. Also try asking students who are ahead of you in your program, if applicable. Some universities also have Facebook groups for current or previous grad students, and you can try posting on those pages as well.
Lifeline 3: If all else fails, email your librarian and see what you can do. Never ever pay for an article, your institution should be able to get it for you. If you still can't get it, try emailing the author of the article, or email me (hmarien@g.harvard.edu and I will download it for you if I can access it.