BTDT, got lots of suggestions:
These days, we get all of ours from the library. Even our small town library has a decent selection for kids, though it's best to reserve what you want or get it as soon as you see they have it in. Check-out time is 4 wks and you can renew up to 2x (for total 12 wks). You can even renew online. The only caveat is that if someone has it reserved, you can't renew. Late fees are 50 cents per item per day.
But for the best selection, we go to the next town over, where we also have checkout privileges. They have 10x the selection (or more! fantastic library system). Everything else applies - 4 wks, 2 renewals, online renewals. But late fees are 15 cents per day.
Both libraries can arrange an interlibrary loan for books they don't have (ie, The Pushcart War). You aren't guaranteed a cd copy (PC is only available on cassette for instance).
Another option is to get them at Cracker Barrel. You pay about $30 when you pick one up, and you can return at any CB. When you return, they keep about $3 for each week you had it. That's pretty cheap for rentals. My numbers are probably off b/c it's been years since we rented there, but we had dinner there last night and they still do books on cd. Kids' books are hard but not impossible to find there. You would do better with YA probably.
http://www.crackerbarrel.com/travel/books-on-audio/ (sample books shown)
You can try ebay, which is good for things that have been outgrown and also good for the free ones that came in Wendy's kids meals for years. They typically sell for 50 cents or $1 (titles like Magic Treehouse, Geronimo Stilton, etc).
When we lived overseas and I drove the kids 1 hr each way to school, we joined Audible. Your first audiobook is free, membership for 1 free book per mo was about $7 (may have gone up), and you got at least 30% off everything else in the Audible store. You download a special program to your pc, download your book in the correct format (for us, pc) and then burn it onto a cd. This only works if your car can play MP3, but most cars can these days. You can also download directly to an ipod and plug it into the car jack.
http://www.audible.com/
As for books - your kids are only a little younger than mine. Current faves for them right now are things like
~the Rick Riordan books (Percy Jackson, etc)
~The Secret Series
~The Mysterious Benedict Society books
~Paolini's Inheritance Cycle (warning: that one is adult fantasy and has a lot of violence ala LOTR).
~ The Hobbit (at 12 and 14 they don't seem to be as interested in the long passages of LOTR)
~ Dragon Rider (this could take your entire trip - my download filled 12 cds)
When they were a bit younger, we listened to the following many times:
~ the How To Train your Dragon series (read by David Tennant and hilarious though full of fart jokes and other tween stuff) (only the first few books are available in the US through
Amazon, but they are sooooo good)
~ My Weird School series
~ The Hundred and One Dalmations
~ Snow Treasure
Good luck and have a great drive.