Luna from Shopify here! Thanks for posting this question. I understand that you still have some questions to address before deciding on an app, so I'd love to provide you with a little bit more information.
Painterest is one of our highly rated print-on-demand apps. Like you mentioned, they have tons of great athletic wear products. To learn more about their company, I recommend checking out the information listed on the Shopify App Store here as well as their website here. If you have any specific questions for them, you can reach out through their Get Support option.
I know that you mentioned you were looking for sporty apparel. Is there any product in particular you’re searching for? If so, I’d be happy to provide more specific recommendations.
In the meantime though, I also suggest checking out Printful here, which is another popular POD service. They offer more than 275 products that you can screen print your designs on. I’ve taken a look at their selection and noticed they have many athletic wear options - including tanks, t-shirts, hoodies and more. They even have a blog post here that includes a guide to selling athleisure and activewear.
When deciding on an app, I recommend looking for great ratings, as browsing the reviews on an app can be a great way to learn more about what the app offers, and how other businesses have benefited from using the app.
I'd love to hear more about your business idea! Are you going to be opening an athletic clothing company?