Dress for SuccessGear are found in blue looking boxes throughout the game, if you look in blue ribbons bar on a table there are 4 pieces, but that is only if you got the season pass. If you haven't got the season pass, they are not too hard to find around the game.
If you want the fastest way to get this achievement, by the time you get your 4th piece of gear you can have the achievement if you do this method. Finding most of the gear in the game is random, so if you hit a checkpoint and find a piece of gear not to far away, reload the checkpoint until you get the piece of gear you need (Hat, Torso, feet, legs).
Here is an updating gear location text guide by Gamersheroes.com:
http://www.gamersheroes.com/2013/03/27/bioshock-inifinte-gui...If you want to look at them simply hit "back" button and RB. This will put you into your "Gear" category. By hitting A on each category you can change your gear if you have more items.