Where do you get the key for the Fort Constantine bunker in the CO Quarters?

The "You Gotta Shoot 'Em in the Head" quest is what you have to start. Ted Strayer (Rivet City, a lot of times in Capitol Preservation Society) has a special key on his body, which can also be pickpocketed. Take Ted Strayer's Special Key and use it on the door. There is a unique set of armor inside. If you already completed the "You Gotta Shoot 'Em in the Head" quest, go to Underworld and see Mister Crowley. He will be wearing the armor