[etadrisophila]

Congratulations on your acceptance to grad school! I shop Amazon (buy used if possible BUT carefully read feedback for third party sellers before buying!), Abebooks, Alibris, Better World Books, etc. Many times, books are cross listed in several seller listings.
Amazon just resolved a problem with a third party seller (who never shipped the book) and I've had other good experiences with them.
Another possibility is to rent the text (rather than buy).
You can go to WORLDCAT and enter your zip code as well as the book titles to see if any local libraries carry them.
Google books has full text versions of various books online. This is not the same as having the book in your hands but if you are short on $$ and the book is available....
Best wishes!
Let's see what the others suggest!