I listen to audio books quite frequently because of a vision problem. I think the most economical way to find them is through PaperbackSwap.com.
It is a national book swap club. You sign up and join then when you post 10 books you get the first 2 credits free. After that you get a credit for every book you send or you can buy credits. Audio books are worth 2 credits either to send or receive.
There are almost 4 1/2 million books available right now! There are thousands of maembers. I heard about this on one of the morning shows and love it!
The sender pays the postage which is usually around $2.50.
http://www.paperbackswap.com/index.php
I have used this swap for quite a while and have never had a problem. There are certain rules you have to follow, like no water damage, etc. but I have gotten some great books from this club including quilt books. If the book is unavailable you can put it on a wish list and when it becomes available depending on your position in line, you will be notified if you still want the book.
They also have a Swapadvd and SwapaCD club you can also join.