I'm pretty sure that nobody reproduces them. Yes, obviously someone makes them, but there has to be a TON of demand to get most of the manufacturers to sell a component of an assembly. We found that out with a couple of items from Dynacorn. Took a couple of years for them to release the core support triangles/body mounts, and several more years to see the hard top cross-braces released individually so you did not have to buy an entire hard top "kit" with all the other parts you don't want.
So while they're being made, I have not seen them sold as a part by themselves. Maybe it'll happen. Best thing I think is to call the various parts manufacturer's sales ends like Dynacorn, AMD (auto metal direct) and places like that. They may, or may not actually own the factories in China that stamp these parts out, but they're responsible for ordering and having them shipped to the US. So that's a start.
Best bet otherwise is to find someone parting out a Bronco or an old rusted out body that still has those in good shape. Some of the under-pinnings were zinc coated at the factory and remain intact while the floor panels rust away to nothing around them. Not sure if those straps were included in that protection, but it's possible.
Good luck.
Paul