I have an unknown maker of a solar tube light. The wife said it was too bright, put some tint on it. So I did and it melted. See picture....
It is a dome-type, about 13-1/2" in diameter. it has a 1/4" lip around the outer rim where it mounts. The mount is a screwless (3 tabs that fit into slots on the tube assembly) retaining/trim ring. The diffuser is about .06-.07" in thickness.
If you cannot find it online (I found several that looked like they would work via google and amazon searches but I couldn't tell from your photo and you did not give exact dimensions), a plastics or lighting shop should be able to order or make whatever you want with built in diffusion or tint that will stand up. Hopefully there is a shop near you or shop online. Theater light gels might hold up to the heat better if you have to put something together yourself.
If you still have the frame and just need a diffuser to fit in it, I should think a plastic shop might be cheaper than buying the whole assembly.
i've tried looking online for an exact match and nada...
I tried using the home depot sheet plexiglas, it warped and started bubbling! I'm going to call the Solatube vendor this week and see if they have something that will fit.
i have more dimensions:
13-12" dome diffuser with 1/4' lip
tube diameter 12-3/4"
mounting ring id=13-14" diameter od=15-9/16" mounting slots@14-1/2"
retaining ring id=13-3/16" od=15-13/16"