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's possible to have this armor almost from the beginning by killing Garl, the leader of the Khans.
Garl, leader of the Khan raiders near Shady Sands, wears this.
It can be looted from a gambler Gizmo's Casino in Junktown. If you have an NPC follower, you can pick the gamblers pocket until he turns hostile, then allow the NPC to kill the gambler. This will prevent the guards from becoming hostile (although other gamblers may attack you).
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.
If you accept the quest from Irwin in the Hub to clear his farm of raiders, a set can be found on the farm. The map cannot be revisited, so there is only one opportunity to grab the armor.