Where can I buy Nook miles?

I'm curious, where exactly can I purchase Nook miles? I've been hearing a lot about them in the cryptocurrency and finance circles, but I'm not sure where to find them. Are they available on popular exchanges like Coinbase or Binance? Or do I need to look for them on more niche platforms? I'd appreciate any guidance you can provide on where to acquire Nook miles and what steps I need to take to do so.