I don't think that they sell it retail as I believe someone gave it to the guys when they made it themselves. I suppose you could always just get a plain black t-shirt and then just iron on white letters to create the design.
I swear, I am not affiliated with these people in any way, except as a customer -- I made the EXACT same shirt online at www.spreadshirt.com, you design it onscreen, they print it up, I had it in two days. I was so impressed after I designed it (using that shot of Adam as a model, ahem) that I ordered two and sent one to my friend for her birthday and she was blown away! It is exact and very affordable and a good quality T.