The only supplements I've ever bought were Glannon Q&A for CivPro and Chemerinsky for ConLaw. I got them both used for a couple bucks off eBay. Glannon publishes newer versions, but heck, there's only a couple exam-worthy things that have changed in the last 5 years on the rules of procedure.
Here's my frank advice:
At first, only buy the main textbook for each class (the one you see with all the readings assigned). Don't buy the supplements, 'suggested readings,' etc., until you know you'll need them (that is, you've been assigned substantive readings in them). I think I still have a model rules supplement, a treaty supplement, some K collection, and a crappy Nutshell book around here somewhere that were wastes.
Try not to buy books that are in your library (esp. ones that are on permanent reserve). And don't ever get a Blacks Law Dictionary; the day you do, someone will give you one as a gift.
Save your money for a laptop lock so you can go take a piss while in the library without risking your laptop getting stolen.
Ivan