Our public library offers 4000+ audio books. I don't know how the "membership" works but you could check it out and then check your local library. Warning about this option--the d/ls are not compatible with iPod.
The only sites I know about are www.LoudLit.org and www.PodioBooks.com. I don't know how appropriate their selections are for your use. A lot of their material consists of older books now in the public domain, and, therefore, free.
There's not a lot of children's stuff per se, but there's a YA section and several titles which are avoiding the "YA" tag like the plague that it is.
If there's something in particular you're looking for, you can actually slide on over to the podiobooks community here and talk to the authors and readers there ... See if somebody would be willing to take on a project for you.
In Kansas, if you have a state Kansas library card you can download them from their website. You can listen to them on your computer, burn some of them to CD, or sync them to your MP3. You might check your state library & see if they have something similiar.
This is a great site, Kevin! The quality, from the ones I clicked on, is so high- really professional. I wonder how I could find the "book" the story tellers are reading from, so I could match up print with text... I guess I could just listen and go through editions (err, I could). Great resource.