I did my search and found only this link, which is broken
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Newer Edit: The shop is closed again :(
There was a store, but it was discontinued pretty fast. They're given out occasionally for miscellaneous things (e.g. the person who guessed when SO would hit 1m questions got one), but I think that's the only way now, they're not sold anymore
You could make one yourself, lots of online places print t-shirts (not sure which country you're in, otherwise I'd pre-Google some links)