I've read in here that softboxes like the Photoflex are lined with something called brimstone. Anybody know where I could get some of this fabric?

I did a search and could not find anything. It almost looks like
Photoflex is making this themselves and are calling it:

" heat resistant Brimstone Technology fabric "

The softboxes are inexpensive enough that by the time you got done making one you'd save very little.

Most of this stuff is aluminized Nomex or something similar. Look up fire-resistant fabric.

I'm not planning to make a softbox, although I do want to try an experiment, if the materials don't end up being too expensive. Thanks Mike.