These are robes worn by Bright Followerghouls. They're very similar to the robes worn by Brotherhood of Steel Scribes, but are slightly tattered and a brownish-green color.
Goodsprings cave - one can be found inside the cave, on the corpse of a dead Bright Brotherhood member who was attacked by coyotes.
Nipton Road Reststop - at the foot of the gas station sign on a dead Bright Brotherhood member.
Primm Pass - one can be found on a Bright follower.
Ranger station Charlie - one can be found on a Bright Brotherhood member laying up against a rock on the west side of the road just south of the station.
Sloan - one can be found on a Bright follower under a railway bridge south of Sloan.
Wolfhorn ranch - one can be found on a Bright follower at the foot of a roadside billboard just north of the ranch.
Another dead Bright follower is found north of The Prospector's Den, and west of Emergency service railyard on a hill. At the top of the hill is a metal electrical tower, the follower is near the base of the tower on the north side of the hill.