By the time you can handle reading, you've learned enough of the language that you can handle searching for audio-books in your target language. Basically, you need to know a good translation for "audio-book" in your target language plus the translations of the book titles you're looking for.
A good way for most popular books would be to go to the Wikipedia page for the book you want and change the language (on the bottom left):
So Harry Potter 1 (The Sorcerer's/Philosopher's Stone) is "Harry Potter à l'école des sorciers" (Harry Potter at the school of wizards), and if you were googling sources to purchase, you'd google "Harry Potter à l'école des sorciers livre audio"
If you can't find it, then you need to start bouncing around to other titles and possibly other authors to see what's had enough mass market appeal to justify an audio book release.
