Can I buy just 1 ear AirPod?

See this, if available in your country you should be able to buy a single replacement Airpod:

AirPods and AirPods Case Replacement - Official Apple Support

Depends of country. Here (Russia) we can buy one AirPod in Apple Authorize Resellers (ex: iPort, reStore). Suggest to call nearest Apple Resellers and ask for them.

You may be able to find a used AirPod on eBay.

https://www.ebay.com/

No.. Not Possible..😃