It's called 4shared.com http://www.4shared.com/

It's got so many brilliant books. Just search on the right hand side. I've even just put in 'jazz piano' and got great results. (Sorry I sound so much like a saleswoman)

Also, pianofiles.com is a good trading website too. You list what sheets you have, and so do millions of other people. You search at the top of the home page , see who has the sheets that you want, email them and you can trade or if you ask nicely, you might just get it sent to you :P

Hope that helps x