I purchased high quality glass convex lenses from Tex Automotive in the UK:
Tex Automotive Ltd. - Richter accessories. They offer 90MM, 92MM & 95MM. I think I paid under $10.00 for each lens. I don't believe you will find them on the Tex website but e-mail Mike Barker and he can help you:
mike.barker@bmh-ltd.com The P/N for my convex Raydyot lense was RGS03. The Talbot style mirror has a slightly different diameter but BMH has that size also.