Try looking on itunes or if not you might have to buy a nook or a kindle. or Google the falowing: the lost hero audio book i have find it for 8.00$ or you can sigh up for audible.com and you get 1...
I don't know of anywhere except iTunes. They're really expensive. You might find it cheaper to buy the discs and import them onto your computer. It's more hassle, but it could save you loads!