Elle here from Shopify's Social Care team. Hope you're having a nice week so far!
I'd definitely recommend checking out the following apps:
Between those three apps you'll be able to find sneakers, backpacks, leggings, athletic t's, sweatshirts, 3/4 sleeves, long sleeves and hoodies.
These apps will allow you to upload your own designs on the products and import them into your store. As soon as you receive an order, the app will notify the company so they can produce and ship the products to your customers!
Hope this is helpful!