Honestly, the library apps are a lifesaver. Libby and Hoopla are what I use for pretty much everything. You just need a valid library card from a participating system, and you can borrow the entire series. The availability can be a bit of a wait sometimes, especially for popular titles like 'The Lion, the Witch and the Wardrobe', but you can place holds. The apps are free and totally legal, which is the best part.
A lot of people forget about LibriVox, too. It's for public domain works, so only the first book, 'The Magician's Nephew', and maybe 'The Lion, the Witch...' might be on there depending on publication dates. The narrators are volunteers, so the quality can be hit or miss—some are fantastic amateur enthusiasts, others sound like they're reading to their cat. But hey, it's free and you're supporting a cool project.
Spotify actually has a decent selection of audiobooks included with a premium subscription now. I'm not sure if the full Narnia series is there, but I've seen some of them. Worth a quick search before you go down other rabbit holes.