I love mental_floss.com. Every single day they post at least one item that interests me, and their content is always of great quality. So it shouldn't be surprising that their online store contains equally witty, interesting items. If I wear a shirt with a witty saying, maybe people who see me will think that I am witty! Genius!
This one is my favorite! $17.97. Check it out; there's definitely something for every kind of geek out there!