Simon, why not make your own? If you have the skill to restore wooden cameras, then you shouldn't have any difficulty making ground glass screens. All you need are sheets of 2mm picture glass and some silicon carbide powder, 500 or 600 grit. You can buy this off Amazon, and any local glass supplier will have picture glass.
Cut your glass to size, or have it cut for you by the supplier. Mix some powder with water into a slurry Put a small amount onto a glass screen, place another sheet of glass on top, and grind in a random circular motion. Wash it off to check your progress and the evenness of the abrasion. Replace with fresh slurry two or three times. Finally, clean with running water.
Alternately you can put the slurry on a pad made from a wet rag, and rub the glass with that. You are less likely to cut your fingers on a wet rag!
I did my first one, a 10 x 8 screen, with nothing more than a 900 grit Japanese Water Stone, used for sharpening wood chisels and plane irons. I got it good and wet, then just rubbed it around on the glass in a regular but random fashion, cleaning the glass off with water from time to time to check the evenness of the grinding.
It is worth putting a finished screen into a working camera to check how well it works and focusses. Then you can tailor the amount of grinding on future screens.
Good luck.
Alan
