In the room with the final bomb on the Station T level, go to the big safe and a covert suit will be lying on top.
Dinner Jacket
Obtain it from Q's Head Quarters at the second part of the Hedgemaze mission.
Easy Kills
Whenever there is a guy that doesn't see you, just aim at him with your pp7 and press X to bring up the focus mode. Aim at his chest or head and fire.
Formal Dinner suit
In the second level, complete the maze and you will be in Moneypennys office. Before you go to M's room, search the drawer to find the Formal Dinner suit.
Bonus Missions...