It all depends on how fast you need them and your budget.
cheapest pre-med textbooks
If you need them in a day or two, then you aren’t getting the cheapest pre-med textbooks. You’ll probably have to spend your grocery money, harboring passive-aggressive angst towards your school, the corporate world, and human beings in general, when you buy them from your school’s bookstore.

Obviously, if your professor put together a course packet for the course you’re in, you don’t have much of an option. On the other hand, no matter how much he or she insists on your getting a new edition, I’ve always refused and never missed out on points or good information.

—Check and see if your school has an online book exchange. Sometimes you can find really great deals and meet the seller on campus the same day.

—If your school’s bookstore is out of stock on whatever you’re looking for, you can try local bookstores like Barnes and Noble.

For the cheapest pre-med textbooks…

—In my experience, you don’t find used books any cheaper than what you get on Amazon.com and/or Half.com. The only downside is that you have to wait up to 14 days (assuming your seller isn’t irresponsible/shady enough to completely “misplace” your order), or you have to pay extra to get quicker shipping. Even then, I’ve had to wait waaaaaaay longer than the expedited shipping time I paid for, and there’s nothing I could do.

—If you’re looking for fiction, go to the thrift store. They’ve got most of the classics and more.

E-books, textbook rentals, et al

—Usually your professor will let you know if there are electronic versions of your book. Those are usually somewhere around half the price for a hard copy, and, obviously, easy to carry. They could indeed end up being the cheapest pre-med textbooks, if you’re lucky.

—If you’re in a literature class and have some sort of e-reader (like a Kindle or a Nook), you can always find the e-versions of books relatively cheap. Not a bad way to go, and I speak from experience. Kindle II is outstanding for books and PDFs.

—One option that’s getting to be pretty popular is textbook rental. Take some time to look around for textbook rental companies in your area, if you’re interested in trying it out. In my experience, most college towns have at least one, if not several.

—Lastly, you can even get hard copies and e-books from online book rental companies pretty cheaply and easily. Many times they offer frequent customer discounts and fairly decent buyback rates, so if you’re at the beginning of your pre-med journey, it might be a good idea to look into it sooner rather than later. I’ve heard good things about Bookbyte.com, but I know there are a ton of similar companies out there, so shop around some and see what you can find. BetterWorldBooks.com does essentially the same thing but promises some sort of positive impact on the environment and society (what that is, I don’t know). BookRenter.com is focused solely on renting out physical copies of books, with free shipping there and back.

**Speaking of buybacks, school bookstores are probably worst deal you’re going to get. They’ve usually got more people than they need trying to sell books back, so they don’t have to compensate you as well in order to turn a profit. If you’re smart, and don’t mind spending a few minutes on it, you can probably take the cheapest pre-med textbooks you carefully shopped for, and turn them into a profit by placing them on Craigslist or Amazon for sale.

The Moral of the Story:

Don’t get hosed by textbook companies. Buy used books or rent them, and sell them back if the chances are good you won’t need them again. In my opinion, it doesn’t make much sense to hold onto pre-med textbooks for MCAT prep, even if they were the cheapest pre-med textbooks! Your textbooks go into far greater depth than you’ll need for MCAT review, which can—sadly enough—make the MCAT confusing or frustrating sometimes. You’ll be able to find MCAT prep books for far less than what you’ll get when selling them back.