Derek, sometime ago, rather a long time ago, I did see ONE vendor that had them. I have looked at the "regular vendors" and have not found one.....but I haven't given up...yet!
....your best bet, if you have not already did it, is to post an ad in the Parts Wanted Forum for an old vent window that someone has around with a bad glass in it. They are around and can be found.....but it may take a little while. Don't give up!
EDIT: it may be worth a phone call to Jim Carter and ask if he has an old one.
Also, Mongo, who is a member here, may be able to help. I think he has some parts trucks that may have one.
EDIT: #2: I do see the entire vent window frame at Classic Parts.....black with the seal $124 for a Pair.....I think the Chrome was 120 a pair. (if you gotta you gotta) but try the swap meet first!!!
https://www.classicparts.com/Vent-Glass/products/859/2/0
