Can I buy just one AirPod?

Yes. See the following from How to find your lost AirPods, AirPods Pro, or AirPods Max - Apple Support

If you still can't find your AirPod or Charging Case, or if something is damaged, get the serial number and then contact Apple Support for a replacement. Then learn how to set up your replacement.

You can also learn more about AirPods pricing and service options.

Hello,

I think this link will help you https://support.apple.com/en-in/airpods/repair/service