Before you try to find the costume, locate the wrench that is laying on the ground. Then, proceed through the level until you reach the man who is shooting from the top of the building. Once you kill him, get to where he was. Then, run across the rooftop to the vent. Exit the room you fall into. If done correctly, the icon "Bash Door" should appear in the top left corner of the room. Bash the door and the costume will be in there.