It can be found in a room in an average locked floor safe on the second floor of the abandoned apartments (green door, to the left if you are facing Haven) under a table in a floor safe with average lock in the Uptown portion of the Pitt.
ps3pc While wearing the Hat of the People, the Pip-Boy lists its effect as "combat jumpsuit Small guns +5". [verified]
ps3xbox360pc If the player collects the item and returns back to the Capital Wasteland, and then un-installs The Pitt add-on, the item will remain in the game, despite the removal of all other Pitt items. [verified]