@Moon it's rare(to almost impossible) to find it as "merch" or Fandom but I only buy pure cotton shirts as besides having allergies to most t-shirts made of plastics(gym and nylon ones I don't get itchy and red) and that cotton isn't as hot and doesn't shrink on washing.

Depending of the brand cotton t-shirts aren't too much more expensive(around to 20-50% more here) so it's an investment that I like. So you could try one to see if you would like it