Gentleman's Attire / Lady's Finery Load up a Singleplayer game to unlock. (Can't be Undead Nightmare)
Posse T-Shirt Load up a Singleplayer game to unlock. (Can't be Undead Nightmare)
Yellow Rockstar Logo T-Shirt Open a chest upstairs in the attic at the Marston Farm house at Beecher's Hope.
Black on Red RDR Logo T-Shirt Open a chest northwest of Riley's Charge in a burnt down building.
Sombrero Shoot somebody's hat off.