I tracked down the audiobook after reading a piece about its impact on true crime. Audible carries it, narrated by Scott Brick, I think. It's a solid performance, though some argue Brick's delivery is a bit too polished for such grim material.
My library had it available through the Libby app, which is how I ended up listening. That's definitely the most budget-friendly route if your local system has a copy. The prose is so meticulously constructed that hearing it aloud adds a strange, almost hypnotic layer to the horrifying facts. The flat descriptions of the Clutter house after the murders hit differently in audio.
I'd warn you, it's not easy listening. I had to take breaks. The quiet, detailed buildup makes the violence, when it's recounted, feel startlingly immediate.