I have a t-shirt that says “World domination is such an ugly phrase, I prefer world Optimization” with a really cool design, and a separate t-shirt that says “Optimise everything” with a picture of a galaxy, and use to have a third t-shirt that says “Probably a member of the Bayesian Conspiracy”. I think Ray also has a t-shirt that just says “Belief As Attire” which I think is excellent.
I got my t-shirts from the people who made PrettyRational.com, but I think it’s shut down now.
We’re planning to make LW t-shirts at LW (have tried some already that I don’t like).
CFAR T-Shirts and Effective Altruism T-Shirt seems to be the closest for people who want to signal rational tribe allegiance.
I think some of the sayings that come from LW and surrounding community would work well on t-shirts. I would both be interested in making and buying them.