I get our audio books from www.audible.com or www.amazon.com. Neither are cheap but they sure make the miles go faster or they are a great alternative to bad TV. I have been a fan of Audible.com for many years. The way to make it cheaper is to join one of their plans. I am currently in a 2 book a month plan for $20 some dollars. I will stay in it for awhile until we get an accumulation of books we haven't listened to and then I will cancel. There isn't any contract so you can jump in or out as you want. I transfer the books to our Mp3 player and then we can listen to them in the car or motorhome by using an adapter. Newer cars might have the connections needed and you wouldn't need an adapter. The adapters are not expensive.
You can also go to the library to get audio books. Cracker Barrel also has an audio book loan program. You pick up a book at one Cracker Barrel and when finished drop it off at another.
Some people have trouble adjusting to listening to audio books and it can take a bit of practice. We always turn it off when we go through cities and construction zones.It is worth the work to get started!