Shadow Archer Cuirass: is found if you side with willy against the assassins, and kill them.
Shadow Archer bracelet: was found in the jewelery box in the room where Abbott was trapped in, inside the forgotten crypt.
Shadow Archer pants: were in a chest in a small treasure room in the temple of nimir
Shadow Archer gloves: were in a chest in the healer house, after mind reading the guy lying by the door in the bed.
Shadow Archer helm: is sitting in a chest in source square, more specifically right as you enter the courtyard where the Engineer is fought. The chest is sitting on a platform on the rightmost wall, just as you enter the courtyard. Running up the platform the Engineer is on and then platform jumping is required to reach the chest.