Farming scrap iron is best done in counterweight east and west in marleybone (inside the royal museum).. These are instance dungeons, spawned solely for the player entering, other players entering the same location cannot "steal" the reagent because they are in a different instance. Also there is no realm jumping required. Each of these two instances has two locations for scrap iron plus one location for mistwood on the entry platform. Randomly sometimes none spawns, sometimes each spot has 1 or 2 scrap iron, occasionally a spot spawns a spring. Once you've cleared the entry platform just leave and reenter.
