A tattooed Monk with the Crab (?I think) tattoo and four other tattoos gets 10/+x, where x = his CON modifier if positive. He can also take Greater Resiliency if he has a class with the Damage Reduction feature (Dwarven Defender, Templar, Sohei, or Barbarian, though Barbarian alignment clases) and boost this by +1 to 11/+x.
The Skin of Iron is a psionic item (not even an artifact) that gives you a continuous Iron Body (psionic power) effect which is similar to the spell except: you can suffer ability score damage from psionic combat only, you are not immune to electricity, and there is no arcane spell failure or amor check penalty. You can also "remove" or "put on" the skin as a standard action.