Are you attending the conference Moduli Spaces and Moduli Stacks in May? Or will you come by Columbia University in the next few months? If so, then you can order a Stacks Project T-shirt and pick it up here at the math department.
The picture shows me wearing the T-shirt; on the back it has “The Stacks Project” in white letters. Since I am trying to avoid spam orders, I will not put a link to the order form here, but I suggest you follow the link to the moduli conference above to find the link over there.