Thanks to all for the useful contributions. I could indeed make them myself with a core drill or carborundum, and polycarb or acrylic are the fallback position if all else fails. However, I was very surprised how difficult they are to find commercially. I had assumed that since there are so many torches, kaleidoscopes etc about, a Google search would easily turn up a source of supply but it appears not.
Anyway, as always the group was helpful and I am grateful.