I just found this site: www.audiobooks.com
You can download to a device (not sure which because I don't do this) or listen from your computer (must be hooked to internet). It is expensive ($15/ month for one book; addition $12 for more than 1 book a month) but so far it is the most reliable I have found, and I have been looking for 6 months! They also give 30-day free trial.
I tried Audible (amazon's site) and you had to down load some crap that would not run on my Win7 machine, and the customer service provided was so horrible that I bailed after 1 hour and 0 books. I wrote a letter of complaint to amazon and their response was "Oh, we don't run, we only own it, that site so we don't care" I will never attempt audible again.
You can find books for free on YouTube, but again you have to be hooked to the internet and my experience has been 50/50. About half of the books were complete and the other half were either taken down at some point or were missing the final chapters. Yes, you invest 6-10 hours into a book and then cannot listen to the ending!!!
And finally I did find a site that sells books for $7-$8 each. They are downloadable in mp3 format. However I think it is bootleg because the quality is sometimes bad (and the books are cheap!). They do not have a lot of books, no kids books, and generally do not have the latest books. They specialize in what popular adult authors like Stephen King and John Gresham. I am hesitant to post this address because I have my suspicions that it is bootleg and do not want to cause any trouble. PM me if you are interested.