is there any good sites that sell cheap used books?

There's a site called bookmooch.com. Basically, you enter books onto your page that you're looking to get rid of. You also enter books you're looking for. Other people also do the same things on their pages. Eventually, when there's crossover (someone's getting rid of a book that you want, or vice versa), you're alerted and you either send the desired book, or claim the book you want. Either way, the cost to you is always only ever the price of a stamp or the price of postage. Hope this helps.

As already mentioned ebay or amazon have every book going most of um can be bought used too. Plus your local charity shops, garage sales and car boot sales are all worth a good look.

Try out sites like Ebay and Amazon. You can find used books and new books in whatever price range you're looking for.