I found some Libbey 8089 Sherry glasses on an old Frasier forum and amazingly they had them on amazon. They look pretty close to the ones Niles and Frasier use and the size of the glass is about the same size too. They came in a pack of 12 for like $53.
https://www.amazon.com/Libbey-Georgian-Ounce-Sherry-Glass/dp/B00BN9IA8Q
Shipper tossed 6 glasses in a plastic bag and put bubble around it. Did this twice for all 12. Only 6 made it to me without breaking and I got a full refund so it all worked out... but these glasses are really great and are the closest I’ve ever found. They look tall in the pic but they are pretty short and fit about a little more than a shot full of sherry.