I'm pretty sure that we are to assume that the metal comes in part on trade or purchase of equipment. Its not to say that the items are made specifically from what our kingdoms would have on hand as much as when we purchase our troops, part of that money is going to a unseen black smith who uses his own materials he attained from who knows where to equip your soldiers.
I would like to see, and I mentioned this in another section of the forums last night, something along the lines of rare mines or materials that pop up here and there. It would get a lot of people to try and fight across sections of land to get them and set up a supply line so they can attain whatever the rare thing is. Once they have it, they can "spend" it on equipping men in their armies with tougher, rare armor, or something of a sort.
It could even make the person who gets it a bigger target in the community, becoming a larger threat with better equipment, temporary alliances could be formed to swat down the now powerful player.