I've found a company Vacuum Coatings Ltd in London, UK, who will restore existing reflex mirrors from reflex cameras or make new ones. I ordered a 70mmx75mm screen today for a Thornton Pickard Ruby Reflex I'm restoring and they expect it to be ready by Thursday. They will cut me a new one and the final price is £16 ($25) which seems very reasonable.

They offer a range of other services some of interest to us as photographers, for instance they make beam spliiters glass,

Ian