I didn't have trouble finding 1/4 hardibacker myself -- in fact, I
didn't even see anything else at 84 lumber here. Mine is nearly white,
one side with faux 2x2 tiles imprinted (well, really I think it's just
for helping the tile adhesive to stick), the other side is smooth.
Comes in 3x5 sheets, and is cheap. I can't imagine it is rated for
fireproofing, but as an experiment I took a few thin slices and held it
in a gas stove flame for 10 minutes. Almost no damage whatsoever, just
a little surface charring. The stuff eats drill bits, knife blades, and
don't even try sawing it. I mounted it with long screws, with spacers
made from short stubs of copper pipe.

The alternative product to this nice stuff at 84 lumber was a kind of
brittle, ugly, bumpy, grey/brown/lint-colored cement stuff, used for
the same under-tile purposes.

Good luck,
-Kevin