Try using Torrents po boss. Just be specific what on what type of file you're looking for, is it pdf, epub, mobi? I'm not sure if there is a specific file for ipad. Generally, google searches are always useful, there is no one website where all books are available. You can also try kat.cr which is a good torrent site. Happy searching, good luck.
Btw boss there is a specific thread for audio books and ebooks, your thread is in the wrong section. You can move your thread there, or you might get warning from admins.i tried searching btw, I can't find a site for it.