OK, I'm probably the last person in the world to figure this out, but Cafe Press has a catalog of roughly one gazillion linguistics t-shirts. OK, mathematically, it's only a few hundred. And not all really that directly 'linguistic'. But lots of them are.
Lots of pretty familiar slogans ("I survived the Great Vowel Shift", "Old linguists don't die … they just leave a trace", "fck vwls"), but there's some neat stuff from the Speculative Grammarian and various other sources (generally unclear). I've taken the liberty of posting a couple of graphics here, for yucks. The IPA eye chart is pretty nice – anybody know where that comes from? (Looks at most vaguely familiar.)
Many of these would make fine Hangul Day presents.