Metal armor serves as one of the two early-to-mid-game armors in Fallout and Fallout 2, along with leather armor. Metal armor has a higher amount of resistance and threshold bonuses but leather has a higher AC. Overall, the metal armor is better than the leather armor. Metal armor is also very heavy for the relatively low amount of protection it provides. Notably, metal armor (and metal armor Mk II) has a very high laser resistance, which is only surpassed by T-51b power armor and Tesla armor.
In Fallout, it is possible to get this armor almost from the beginning by killing Garl, the leader of the Khans.
A gambler by a slot machine also wears a set. With a companion, it is possible to purposely get caught pickpocketing him into hostility, then allowing the companion to take the kill. This will not provoke the guards (but may make other gamblers attack).
It must be looted before completing the quest to take out Gizmo for Killian. The gambler and everyone else in the casino will be gone after killing Gizmo.