a local fireplace supply company to see if they can custom cut a piece of glass for you.
Once the company is gone you run out of options. You could also see about something like Craig’s List or some of the other local sites like that. Someone may have one in the garage or basement you could buy for cheap.
It’s possible, but not likely, that a local glass company could find the proper glass and cut it for you but I’m betting it wouldn’t be cheap, but it might be worth the cost of a phone call to find out.