I've been reluctant to post this, but here goes.
If you don't know what a torrent file is, it's time to learn. Go to
BitTorrent.com and read about it, and download their software.
Torrent files are small text files that allow the software to find data on the web. By "data" I mean computer files in the form of software, music, and yes, audiobooks.
I've known about torrents or quite a while, but it seemed like I could never get them to download anything. Maybe I was choosing bad files (some will never work), or maybe there just weren't enough other users at that time (it's P2P and relies on user participation), but it works great now. I've downloaded about ten gigabytes this week (no joke).
From the BitTorrent site you can type in a search query for the titles you want, or you can go to
The Pirate Bay, where there's a category devoted to audiobooks.
Bear in mind that not all of these books are in the public domain, so you're on your honor to download legally. They have all the Harry Potter books, as well as Douglas Adams books. In some cases, you would be legally allowed to download them. I trust you to do the right thing.