If you want to dress in the spirit of Red Dead Redemption, if not literally in a tattered hat and poncho, Rockstar has you covered with this official flannel shirt.
It's a collaboration between the developer and label Barking Irons, which sees a limited run of the red & black shirts available on Rockstar's website for $55 each. A tad pricey, perhaps, but cheat at a few rounds of Liar's Dice and you should be able to cover it.
Red Dead x Barking Irons Flannel [Rockstar Warehouse]