Towne Ferroxyplate (908) 722-9500 in Somerville NJ makes 4" x 4" x .060"
glass coated with S1800 resist and iron oxide.
My 1200 dpi laser printer makes mask transparencies good for large features
(~0.1 mm).  Better printers (more dpi) work better but cost a lot more.
Good luck,
Marty