They aren't SUPER weight bearing and I would guess 15-16 ga. steel
I got mine from buying a parts lot grab bag on Ebay... cleaned em up and painted them with wrinkle paint. This was the same way I got one of those keeper springs and mounting tabs for the license plate flipper McThingy. I've always wondered why the after-market stainless ones don't come with any of that hardware... the Bronco mysteries continue to baffle and cause much head scratching.
I would imagine that you could check with
Bronco Hut or some of the distributors that mainly deal in used parts from tear downs. Heck put a "Parts Wanted" ad on this site and I'll bet someone has a pair that they would send you a set for a couple bucks and shipping.