To purchase pro sports and comics t-shirts, you need to find licensed manufacturers and distributors. You can probably do some Google searching to find them. In the case of sports shirts, perhaps you can contact the leagues directly for a list. The big ones are Nike, Reebok, Majestic, VF, Mitchell & Ness. I'm sure there are others though. I don't know as much about comics, maybe someone else can help.
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