Let me tell you a little secret: Jason and I don't like spending money on textbooks. We seriously, seriously dislike it. In fact, Jason and I have the goal to make money on our textbooks each semester. And we usually do, or come close to breaking even. Some semesters are more difficult than others, and sometimes you just can't, but you can seriously decrease the amount of money you're currently spending on textbooks. For example, last semester I spent a total of $14 on textbooks. And that's with two science classes, two math classes, and a history class - all with required textbooks. If I had bought my books at the bookstore, I would have paid several hundred dollars. But I didn't! And I really want to share all our tips with you.
Also, I realized I probably should have written this like two weeks ago when it would have been more help to all my BYU friends. BUT, it may help you in future semesters.
So, without further ado, here is me and Jason's guide to where to buy your textbooks for BYU:
First of all, the BYU bookstore is absolutely the last to buy your books. Seriously, the last. They always have the highest prices.
Secondly, get comfortable with older editions than the ones specified for the class. It may be scary at first to have the 6th or 5th edition instead of the 7th, but they usually work out just fine. They just have different page numbers and sometimes slightly different organization. Use your judgement, but be okay with going out of your comfort zone.
Places to Buy
The very first place you want to look is the library - both the HBLL and the Provo City library. The HBLL actually has quite a lot of textbooks (albeit, usually older editions). A lot of times you'll get lucky and they'll have exactly what you need, or an older edition. It was because I checked nearly all my textbooks out of the library last semester that I was able to spend only $14. Make sure you check them out the first day of the semester (or before) and that'll guarantee you have them for three weeks; if you can get a spouse or friend to put a hold on them as well, you'll be guaranteed six weeks of the book, and that's usually enough to guarantee everyone else will buy their own copy and you can keep rechecking it out all semester.
Secondly, go to DI. Checking out their textbook section requires patience and craning your neck at odd angles for periods of time, but it's worth it. Jason and I have both found several required textbooks there. Instead of buying a $100 calculus book from the bookstore last semester, I got an older edition for $8 at DI. I had to make sure the problems were still the same at the math lab, but it was worth it. A bit of work = money you can save.
Next, check Amazon.com and the BYU Book Exchange. Sometimes Amazon will have your book for cheaper, sometimes Book Exchange will. If you use Book Exchange, you meet the person on campus and buy the book physically from them. It takes a couple emails to sort it out, but again, it's worth it.
Another option might be regular bookstores like Borders or Barnes and Noble. The list price of your book will most likely be the same as the BYU Bookstore, but if you have a coupon from their email mailing lists you can sometimes get one or more of your books for 30 or 40% off.
Lastly, if you seriously can't find your book anywhere else, you can go to the BYU bookstore. If you need to buy a packet they're usually the only ones that carry them; but even those you can sometimes find on Book Exchange. If they have a used copy of your book, great. If not, I feel your pain, and I'm sorry.
Places to Sell
And, one last tip: after the semester, don't automatically sell back your books to the BYU bookstore. You can usually sell them for more on Book Exchange or Amazon. Check those places out first!