I actually bumped into this because I burned through my Audible credits. 'The Chamber' is on Audible, obviously, but I was looking for a free option. My local library uses the Libby app, and I managed to borrow it from there without a wait. The narration by Arliss Howard is pretty solid—he nails the grim Southern atmosphere.
I’d say Libby is the first stop, since it’s free if you have a library card. Audible is reliable if you’re a subscriber or want to buy it outright. I’d steer clear of random YouTube uploads; the audio quality is usually terrible and they get taken down fast. I listened to most of it on my commute, and the legal routes just make the whole process less annoying.