It can be found at the bottom of a staircase in the Sierra Madre vault room.
This job doesn't pay enough, working down here with those walking lightshows gets on my nerves. And the elevator... does Sinclair know how dangerous it is to have the elevator lock like that? If someone accidentally trips the deadman's switch inside his Vault... even Sinclair himself... this whole place becomes a big metal coffin. Lou tried to tell him and Sinclair said it would be "user error."