I have to ask, is this even a problem anymore? I just moved to 1.5.7 and so far it seems like every armor and most if not all weapons showed up in the game pretty early. I don't think I have any mods that alter this or at least the mods I am using that I think might actually be able to alter this, didn't in previous versions. If it is a mod, well then Taleworlds needs to figure out which one it is and copy whatever makes the armor show up. Oh and the prices are reasonable as well. Best chest armor sells for about 58k. Best helms, 18k.
The only thing I don't see being available is actual horse barding. I see plenty of harnesses for sale but no actual horse armor.