As n7bsn said, Google free audiobooks. Or get on Audible, which usually has better quality.
If you're a Christian, you'll appreciate that ChristianAudio.com gives away one audiobook every month on their e-mailing list, and most of the time it's a good one, worth keeping. Their stuff is all human-narrated, not computer.
Doing these things, I've got more audiobooks than I can possibly listen to in the near future :)
Something I've wanted to try, but haven't, is to use a quality text-to-speech converter and put other ebooks in audio format. Here's a review of the top 10:
http://screen-reader-software-review.toptenreviews.com/I once listened to an audiobook with a really bad text-to-speech. It was rough going for the first two chapters but after that I grew accustomed to to the weird choppiness. I'm sure they've only improved the quality since then.
Also my public library offers free digital downloads anywhere you have internet, but I'm not certain if they offer audiobooks. Yours might? Worth checking.