I'm probably getting too old for subtly ironic/clever/geeky T-shirts, but *%$# it. If I have to be a responsible parent and taxpayer, no one's taking away my Barack Skywalker tee.
While there are many fine outlets selling this kind of apparel, one of the best I've seen is Red Bubble, which is sort of a cross between Etsy and Facebook. You can buy and sell artistic wares on Red Bubble, and you can interact with your favorite creators. For my money, the graphic artist-created T-shirts are the real stars of this site. Here are a few of my favorites:
Steampunk R2-D2 by Reece Ward
Who Is Watching by Warren Hart
Star Wars Coffee by rubyred (I love this guy's stuff and bought this one.)
The Sarcasm Foundation by digerati
How We Roll, That Is by James Lillis
There are many others, and I haven't even mentioned the many art prints. I'm not being paid to pimp Red Bubble. It's just cool to see independent, creative types flourishing in a lousy economy. Plus, I don't want to go broke alone.