I always thought western shirts are the easiest things to find. any decent vintage store will have tons, and I find most of them to be very slimfitting. IME, good short sleeves are harder to come by than long sleeves, which are dime a dozen. The only time it gets hard to find is if yer after the good shit, which is pre 60s rayon gabardine, generally HBarC but MacMurray also made nice ones. Those are the sheeznit.