Here are a couple of places where you can design a shirt and not have to order any – you just get a portion of the revenue — if they sell. They are not printed until an order is made:
Sell your own T-shirts: Use https://www.zazzle.com or https://www.threadless.com or https://www.cafepress.com/
Check out my favorite sarcasm shirts here: DespairWear
Sell your idea to a major company:
One Simple Idea: Turn Your Dreams into a Licensing Goldmine While Letting Others Do the Work by Stephen Key