Give the tat artists some of the shirts. They like to show off the artwork that they have done and people waiting in a tat shop usually skim through the tat book, look on the wall at the designs and just try to distract themselves to kill time, seeing the shirt would likely have a better impact while they are in that environment over say a facebook post or a website ad. Easy way to get free advertisement that targets people interested in the artist. The fact they are tat designs adds its own gimmick which will bode well with the tat addicts that go there (some people just turn up to chill with the artists).
Will be hard to get it off the ground but its relatively easy to keep on top of.
Good luck to ya bud.