Many rally participants asked where my 2nd Amendment & motorcycle t-shirts came from.
I re-discovered the link....

https://teespring.com/shop

What they do is take an idea, mock it up, put it on sale for a certain period of time, and if enough people commit to buy, they make the production run and ship the product.
Really high quality t-shirts too....
I do not know if they ever do an encore for a successful design.
Cheers,
Steve

PS - I have no fiscal interest in this company. I just REALLY like their products.