I've heard good things about teespring. I gather it's like a kickstarter but specifically for t-shirts. They seem to have some procedures[1] in place specifically for non-profit organizations, which might be a good option for supporting
haskell.org.
[1]:
http://teespring.com/solutions (confusingly, the page is titled "enterprise solutions", but I think it's actually about non-profit organizations)
Assuming the campaign isn't hard to set up, it's probably worth doing. I know I'd certainly buy a shirt. Or maybe a sweatshirt.
A campaign like this might also make a good rally point for informing the community that
haskell.org is now a registered non-profit :).