Buying textbooks online
OK, here is a not very carefully compiled list of online places to buy or find textbooks. This list is not comprehensive. I'm not recommending any of these sites. A lot of these are comparison sites, so there will be a lot of repetition. You might save money and/or time, or you might not. I generally find that buying from actual bookstores is more expensive but the shipping is faster. Buying from individuals can mean excellent prices, but delays in shipping, no matter how much you pay for "express shipping". (It doesn't get to you fast if they take 2 weeks to get it to FedEx.) Be sure to check for ISBN's, editions, and correct authors (there are hundreds of books called "Microbiology"). You might want to ask your professor about using an older edition.
And, of course, it never hurts to check your local libraries; just remember that most libraries do not buy textbooks by policy, though they may end up with some as gifts or because it's a generally useful book besides being used as a textbook. You can also ask your professor to put a copy of the book on reserve (assuming that "I can't get to campus" isn't the reason you are reading this.)
All that said, here's the list. Feel free to add other recommended sites in the comments.
Ichapters.com (online ebooks by chapter)