Buy shit off amazon, bookburro book mole etc a semester in advance. I routinely snag $300 textbooks for 20 bucks, review books for pennies. Generally an edition or two behind the current version doesn't hurt anything. Google the shit out of the ISBN number, title, authors. Usually you can find a PDF if you try hard enough and it's a popular enough book, even though you might end up with a virus.
Also, talk to people who have taken the course to find out what you really did or didn't need.
Asking the professor really doesn't work that well. Once I took a stupid fucking required archaeology class (WTF, UT, WTF) and there was like $800 worth of required textbooks. I double checked with her the first week, and she insisted they were all required in order to pass etc. We ended up using none of them, not even referenced once during the entire semester. I managed to get them all for ~300 somehow, really wrecked my budget. Still mad about that.
I still hate her eight years later, stupid fat whore.