This plaid shirt is mostly white with red and blue strips in it. The red and blue are emphasized on the vertical and dimmed on the horizontal. The plaid runs diagonal on the pocket and shoulders, and the whole thing has white snaps, not buttons.
Brand: American Eagle
Sam first wears this shirt in Heaven Can’t Wait (9x06), at the beginning of the episode when they’re researching in the Bunker and Dean gets the call from Cas and takes off.
In Inside Man (10x17), Sam doesn’t wear this shirt, but we see it hanging up in his room as he reads Bobby’s letter at the end of the episode.