A lot of times you can find books for 10 cents- a dollar a piece at yard sales. Occasionally I have even seen people give books away.
Ebay is a great source for really cheap books. If you look in the wholesale category you can even find pallets or truckloads of books at very low prices.
A lot of times libraries will hold bargain basement sales or feature some overstocked books for sale for low prices when you first walk in.
Some used book stores offer books for a dollar or two a piece or offer some at half price from what is listed as cover price.
There are book clubs that you can join that give you books at highly discounted prices if you agree to buy so many per month or year for a given period of time.
select one here...