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
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Who Is Watching by Warren Hart
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Star Wars Coffee by rubyred (I love this guy's stuff and bought this one.)
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The Sarcasm Foundation by digerati
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How We Roll, That Is by James Lillis
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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.