Go to Marleybone and head to the counterweights in Big Ben. Once you enter, there should be either some mist wood or iron. Grab and exit. place a marker so you can teleport back to the door. Now run over to the other side and enter. Grab the iron in that room and exit. Now teleport back to the first door, rince and repeat.
i had the same problem but once you quest enough in marleybone and you get to the counterweights, enter one, enter the other, realm hop, repeat. i got all my scrap iron while questing in mb but once you are done do the counterweight method. otherwise just keep on questing and you will find it. hope this helps!