

There’s a burnt down and abandoned manor about 10 minutes walk from me, now owned by the Freemasons, has a hermitage carved into the sandstone (or limestone idk) cliff face, as well as a tunnel leading from the local castle to the manor as a means of escape (something like 1.5 miles).
The hermatige is pretty small, about 10x10 foot in the main room, with a super high 15-20ft ceiling.
There’s another mystery cave carved into the cliff, which is about 30x15 foot with pillars supporting the ceiling carved out of the rock.
The manor has been used in one form or another basically continuously since the crusades, and then intermittently before then, earliest record is a Roman reference to a spring around the location, there is also a Saxon era water mill
Absolute worst case scenario if you are unable to get a replacement, you can get a pane of glass and either score and cut it yourself (diamond or silicon carbine and a ruler to score, and then either a drop of water on the score lines and snap or put a bit of string soaked in lighter fluid and set it on fire on the score line, once it’s heated dip it in cold water or pour cold water on and it should split along the score).
Use youtube to actually see how it’s done.
Or go to a window maker (or if your hardware stores do it) and ask if they can just cut a pane to size for you.