Hi Matt,
I have a camera like yours. They require a special plate holder with a thinner than standard rim (called Normalfaltz in German). Zeiss, Voigtlander and many other holders are too thick to fit KW Patent Etui (and could also be slightly wider). You will need to find the original KW holders.
There's a whole thread on 9x12 cm holders here:
https://www.photrio.com/forum/threads/9x12-plate-holders-1.39756/
You should be able to use 9x13 cm photo paper in the film pack adapter that you have. Put the paper under a black card with some foam to work as a pressure plate.
Foma produces sheet film in 9x12 format. You will need a sheet film adapter to use it in a glass plate holder. They are not very common but do appear sometimes. Before I got sheet film adapters I used to attach the film to a scrap glass plate using a few drops of syrup. I also found that CD/DVD cases make perfect plates to put behind the sheet film.
As you've discovered, new glass plates are also available both in the US and in Europe.