I would imagine that when the time rolled around for Radio makers to find suppliers for the Dial glasses for radios, the clock industry were already being supplied with Dial Glasses/Faces for multible millions of clocks. And those suppliers would have become suppliers for Radio Dials also. I would also have thought that these Dial Glasses were available in standard sizes. This afirms my thought that those Dial Faces, Clocks or Radios used those same glass products. Both then and now. As a matter of fact if you Google Clock or Radio Dial Faces, you will find many such glass suppliers, still today pushing their dial glass products on-line. Try Amazon, Alibaba, or eBay for starters. Personally I would check out the Second Hand Stores. You could take the remenents of the broken glass with you. If you still have the peices, tape them together and use them to check size and curvature before purchasing an old clock.
Also once you find a suitable replacement lens, set it in a few dabs of clear silicone, and if you tighten it up with clips, do not over-tighten.
Lots of luck in your search.