I recommend farming the green Men in Wysteria, or the Death Oni in Mooshu. If these aren't options you're interested in, many O'Leary mobs in Marleybone drop iron, as to the Sokkwi in Krokotopia. Troubled Warriors in Grizzleheim are another option.
If you don't want to farm mobs, then farm Stone Blocks by realm hopping in a place you like, I prefer Dragonspyre Academy or Shadow Palace, and transmute Stone Blocks and Opal to Pyrite and then the Pyrite and Opal to Iron.