Hi, I am a new(-ish) player who is currently going through the Barbarian campaign, and I've run into the problem, that while I can find tons of weapons (sometimes 8+ on a single map) I can barely find any armor for upgrading units, meaning that the majority of my units are either Bersekers, or Stagriders.
This has made some of the more defensive fights extremely difficult, since while I can dish out a decent amount of damage, i can't really take any in return (especially ranged damage from siege golems and longbowmen has obliterated my army more times than I care to count).

Currently I have the choice between going to Iseal, or the other borderclan, that has all the iron mines. Will going to one or the other influence what kind of upgrade resources i can find?

You made me uninstall this game a 3rd time, with bugs, unbalance and gameplay errors.
I had hope

a bit late for an answer but
1. yes the iron guys are the blacksmiths of the tribes they have the armor, the other guys give magic tho so pick your preferred resource
2. apparently when looting villages and exploring ruins, caves and the like, the loot is random and can be savescummed with an autosave BUT armor is still quite rare AND the loot depends on the wealth bar (turns since start of battle) so preferably get those things as soon as possible (quick units are good for that)

3. you are playing fantasy celts/picts, early game you fight one another, late game you invade the roman empire. That is when you get armor and magic
4. Apparently the devs realized this problem and in the empire and onslaught campaigns you have access to merchants who buy and sell stuff (artifacts, weapons, armor, mana, supply) and even modified the main campaign so once you enter the empire you also have access tosaid merchant (needs dlc though)