Try KHB Photographix in Missisuaga Canada. They are the go to guys around here for Omega , but know fixing and modifying things inside and out. Their prices reflect thishowever. They have a thorough web site.
If it is a DIY fix you are after, look to a localish theatrical supply houe (or B&H) Rocso or Lee both make high temperature tolerant diffusion materials with different characteristics available to pick from.
You might also try the art supply store for a sheet of white coreflute.
Coreflute is a plastic version of corrogated cardboard.
I have used it to make DIY grids for studio flash light modifiers with the black varietry of it by cutting thin strips and stacking them on edge. The white variety could be used in a similar manner to mimic the original hex cell diffuser.