Like many of the athletic superhero shirts Sheldon wears on the show, this Superman tee actually also has lettering on the back side of the shirt which reads "MAN OF STEEL". Also like those athletic tees, most replicas of this shirt found online are lacking that back portion. With that being said, you can still purchase a replica of the front portion from the Amazon link found below.
Featured in The Big Bang Theory episodes
S04 E03: "The Zazzy Substitution"
S06 E17: "The Monster Isolation"