Audible is a division of Amazon dealing with audible books. A free 30-day trial is available. Books purchased or borrowed can be heard on any of your devices.
Delphi is a growing source of collection (in Kindle or ebook format). The site contains Audiobooks (ca. 20+ @ $49.00).
10 sources for free audiobooks for iPhone
(most public libraries have similar services)
Audiobooks app