After that I go straight to my top 3 sources and search for the lowest and most acceptable condition possible.
I have bought books for $5.00 that originally cost over $100.
Every book I buy is used, but in acceptable condition.
Here is a list of the three websites I use to buy my books, starting with my favorite.
1. Half.com This is the first place I go to search for my textbooks. Also this is where I end up getting most of my textbooks from. When you go to the website just type in the name or ISBN number of your textbook and you will see the best prices.
2. eBay.com If I do not find a textbook for a good price on half.com, I check eBay's auctions for that same book. Also, even If I find a good price, I also check eBay just in case there's a better price. Keep in mind that International editions are the same book but cheaper. So if you see international edition go for it.
3. Amazon.com First, one thing I like about Amazon is getting free shipping on all my study supplies after signing up for Amazon Student. Secondly, because I find a bunch of coupons from retailmenot, I always check for coupons before I finish my purchase. Third, I buy used and so the price is very cheap. Lastly, being that textbooks can be very heavy, when I buy directly from Amazon I select the textbook to be in kindle format (if available). Amazon is now offering their kindle app for free in different devices, so I find it very practical.
Now, figure out what textbooks you need, do this a few weeks before the semester starts. Why? Because when the semester starts everyone will be looking for the same book, prices may rise and you may not even get the book at a good price. Start early, get the best.