What's the most niche hobby merch you've ever bought?

I was thinking about how almost every hobby has its own merchandise. Gamers have gaming tees, sports fans have team jerseys, musicians have band merch, etc.

Recently I came across some clothing based on speedcubing (Rubik's Cube designs), which made me wonder how many people actually buy merch related to their hobbies.

What's the most niche hobby-related item or clothing you've ever purchased?