I have made tie dye shirts every trip. . .and every single time one of us gets asked where we got them. . .so if anyone wants to make them for profit, you might want a card with contact info on them so interested folks can get in contact with you for size etc. I don't know if this is 'legal' or not though. A year or so back there was something with someone's webpage and license and money, but I don't know the particulars.
A couple of weeks ago Joann fabric had t-shirts 5 for $10, and that included XXL, unlike Michaels, which stops at XL with the sale price. Worth keeping an eye out for if you need bigger shirts.