Just fast travel around to all the cities and buy all the smiths iron ingots. If you leave after that and do a couple quests, you get some more money to buy ingots, and this gives the smith a chance to re-stock. Also, if you look near the crafting tables at each smith's shop, then there's often a pile of ingots there, which I think respawn.
I'm guessing you're trying to power level smithing. Exploring is one of the sure and tested ways. Run around, especially near mountains and look for bandit camps/mines. The reason I say bandit camps is that chests often contain iron ore/ingots there if it's closer to a mountain.
