http://suprbaydvdcaynfo4dgdzgxb4zuso7rftlil5yg5kqjefnw4wq4ulcad.onion/archive/index.php?thread-119640_html=
The best site I've found is audiobookbay. The thing is, it isn't really free. It allowed me to download 1 audiobook, and when I went to download a second, I could go no further until I chose one of several different membership levels, all of which had a monthly fee.