Scattered throughout Willamette will be items of interest covered with cosmetic icons (cap, full suit, shirt, pants, boots, glasses) that are going to be labeled as Clothing Items. The best place to find these articles would be, obviously, clothing stores which are more obvious by their T-Shirt legend icons on the map and can be found more frequently in the mall. Should you have the scrap, you can buy full outfits in the safehouses from the Clothing Vendors (which can be anywhere from one to five pieces a piece).
Outside of the mall, you can find clothing pieces hanging up on hangers on the wall, shoes littering the floor, or pilfer from dryers outfits. All of the pieces can be accessed through mirrors or clothing stalls and if you're wanting to just get credit for the piece, press Circle to wear it, wiggle and try to move to stop the animation short and then press Circle again to remove that and return to your previous attire (there are some instances that this may not be true, as for me when I wore and removed the Cupid outfit it gave me Dark Mecha Zangief).