Dean has a plain brown (or perhaps army-green) shirt that is cut in a western style, with reinforced shoulders and pointed breast pocket flaps. It does up with snaps.
Brand: Levi (Barstow Western Shirt)
In Dark Dynasty (10x21), Dean is wearing this shirt at the beginning of the episode when Sam comes back to the Bunker.
In Into the Mystic (11x11), Dean wears this at the end of the episode as he says goodbye to Mildred, and then when he is back at the Bunker for the final scene.
In Mamma Mia (12x02), Dean wears this shirt at the end of the episode when they are finally able to have a family dinner back at the bunker.
In Advanced Thanatology (13x05), Dean wears this shirt at the beginning of the episode when they are in the Bunker.