Like Bose SoundSport Free with StayHear+ Sport tips, Apple also has their own signature eartips for Airpods Pro.
Just in case you lose Airpods Pro eartips.
You can contact support for a new pair. And it costs only $4 for replacements.
First off, Go to https://support.apple.com/ and then log in using your Apple ID, select: Replace an AirPod or Eartips and follow the step guide to get your new pair.
Currently I haven’t found aftermarket replacement tips for Airpods Pro yet. However they could arrive in the next few months.