I think it was the 30 Rock DVD commentary in which they mentioned that they had to make the shirt themselves because it was not a real thing!
EDIT: I just watched that scene again with commentary, and they didn't actually say it there. I do not have a reputable primary source, but the trivia section on IMDb does say as much.
The Battlestar Galactica (2004) "What The Frak?" shirt Salma Hayek wears was custom-made for the episode. It has since been made available for sale.