It's been a decade since I was in, but my sources were as follows:
1) The on-campus bookstore actually had reasonable prices on used books (I understand this is unusual, they were school run and not for a profit).
2) Where ever Google took me. This was Abe, Amazon, Ebay, and a number of other places. If the professor was going to pull homework from the text version sometimes mattered, if not maybe not but check with the prof. Mine were good about telling us when we could get away with older versions, although often in that case they just made the official course book the old version so I got lucky there.
3) Upper classmen. Make friends with them. I was able to borrow several books from other students who had purchased them the prior year.
4) It wasn't an option at all for me 10-15 years ago, but my father is now auditing courses at Ohio State (he lives in Columbus, OH) and in some cases he can get an official PDF of the course book for $20 through some campus program. In other cases the PDF is much higher, even above the cost of the used book, so you need to check each time.
It doesn't help the first quarter, but be sure to plan online book purchases several weeks out, media mail can be slow.
Interesting side note, there are 5 NCAA D1 schools that go by "Spartans", although only 3 of those have football teams.