Sam’s got this snazzy brown long-sleeved shirt with grey strips that go over the shoulders and down the sleeves. Sometimes he wears a t-shirt over top of it.
In Phantom Traveler (1x04), Sam wears the long-sleeved T when researching and trying to save the airplane.
In Skin (1x06), Sam wears the long-sleeved T after the second attempted-murder, in the sewers, and while fighting Shifter!Dean.
In Home (1x09), Sam wears it while he sketches his dream and confesses his visions to Dean, and when they first visit their old house.
In Route 666 (1x13), this could possibly be what Sam is wearing under his plaid shirt after the third death of the episode (the morning after Dean sleeps with Cassie).