Iron Ingot can be acquired with a Shovel in the Cliffs region of the main Island.
This can be used in the following recipes:
Where to FindEditIn the Cliffs region of the Island, after pushing a rock into the hole between two palm trees on the main screen, you'll be able to move to the east to a screen with many Teak trees, as well as some islands out of reach.
Unlike other items found in the Cliffs, this one does not appear to regenerate. That's ok, since you only need to build a Hammer once.