Instructions
Unlock the Algonquin Island of Liberty City by completing missions in the game's main storyline.
Go to a Perseus store -- there is one in the far south of Algonquin and just to the east of the park in the middle of the island.
Walk around the shop and try on a suit by pressing the "X" button while facing the rack where that suit is hanging.
Go to the cash desk while wearing the suit and press "X" again to pay for it. Most suits in Perseus cost $1,000, but there are a few which cost $2,000.