NOTE: These don't always help if you want new books or you want a certain book in particular. This is for the avid reader who just has the desire to get as many good books as possible for a low price.
I often go to places such as Goodwill or a local thrift shop. they sell books there, often for anywhere between a quarter and a dollar, and other shoppers usually aren't looking for books. You're left with your pick of what you want and they're cheap and in good condition!
The library usually has a book sale table. I five books in almost perfect condition from a series once. They cost about a dollar all together. I also got Divergent from the library for a quarter. Sometimes these racks are bare or almost empty. Ask your library when they'll fill up again.
Yard sales. It seems people tend to get rid of books first. I personally, as a book hoarder, don't understand that. Usually they just want to get rid of them so they're really cheap!
Look around on Amazon. Sometimes you can get books for just pennies (plus that dratted shipping)
If you're a blogger, check this out! You sign up and they review your blog. If you're accepted to be one of their reviewers you get a free book (Which I did, and it's on it's way in the mail! Eek! One I really wanted to read too!) Check it out :D