I farmed my iron for these quests from... thousand needles. there were a few good places which always had some good deposits. I always went to, highperch (2-3 there), the harpy den and just outside of it (2-3), and just along that back wall, i'd always find about 2.
It took a while, but I eventually got it all, and now i'm an armorsmith (well after I finished all the other quests). But remember to save your mithril, you'll need a ton of it.