Hello. My name is***** can help you with your question.
You are correct that they are quite common – if you happen to know what they are called. What this is called commonly is a "glass shade". You may have to do some additional searching to find one that's precisely like that. But what I would do is measure the 2 openings and that will make it a lot easier to find a replacement. Let me know if you need additional help finding it.