Clothing can be purchased from different clothing retailers all around Chicago. All purchasable outfits in the game are a pallet swap of the original outfit Aiden wears, with slight alterations in some cases. They don't affect gameplay and are purely cosmetic. There are also multiple outfits that serve as version-exclusive, depending on which edition of the game was preordered. For a list of outfits in Watch Dogs, see here.
Watch Dogs 2
In Watch Dogs 2, the player can fully customize Marcus's outfits, with any combination they want, a list of outfits in Watch Dogs 2, see here. Marcus can now buy clothing from vending machines and kiosks, but some places will sell different types of clothes then others. When near a store or kiosk, the player can select wardrobe, and sort all clothing they own by brand.