I think at this point any shirts would be produced by the band. I don't think Tzadik makes too many tshirts for their artists (not that Harry Partch "Delusions of the Fury" shirts wouldn't go flying off the shelves at Hot Topic....)
Yeah, Tzadik has a Tzadik shirt available but no shirts for their artists. They don't really operate like that, the way other labels do, having an air of ownership or provenance over an artist that has released a record with them. They're more like something that exists to provide an outlet and help for artists to somewhat self-release singular works.