I picked up a Raygun "Prohman Empire" shirt at HyVee the other day. Local, but not underground. Rueben Tees used to have some decent shirts, but they haven't refreshed in quite awhile (still have Mayor and "Sons of Paul" shirts.)
None are as good as the "I'm so proud to be your shirt" that I picked up from another CFer, in the parking lot at JTS. Felt like an elicit drug deal, just waiting to go bad, with the ISU logo police coming down hard... Funny, I wore that one to oSu one year, ended up tailgating next to some FB support staff that didn't travel w/ the team. They insisted on taking my pic to show coach, that he'd enjoy the humor.