If you are planning on buying them online (i.e. cheap international editions) or don't care about having the newest edition, check out the
course outline for each course from previous years and look up the textbooks that were required last year.
Also, I find
http://www.booksprice.com to be the best place to compare prices of texts online.
Used it to buy most of my textbooks last year (second year bio books) and saved tons.
You can easily get the ISBN # off of Amazon once you have the title, and then on you go...