Threadless and Etsy were two places I was going to recommend. I see others had the same idea. Other places to look are ThinkGeek.com (if you like geeky stuff like computers, science fiction, etc.), and CafePress.com.
http://www.thinkgeek.com/tshirts-apparel/
http://www.cafepress.com/
Personally, I am a sci-fi geek. My boyfriend is a sci-fi geek as well, not to mention a major zombie geek. We are also in our 40's, so we grew up in the 70's and 80's. So my graphic tees tend to be related to favorite science fiction shows, or they have zombie related art, or they have images of candy, TV shows, or video games that were popular when we were kids. (That last category of t-shirts is easy to find in thrift stores. And since my boyfriend loves to thrift with me, we have quite a few that we have picked up that way. He travels a lot, too, and sometimes will pick them up from thrift stores he visits on the road.) I also have some concert tees from my younger days that I've held onto, as well as t-shirts from the science fiction conventions that we have attended over the years.
I wouldn't overlook eBay, either. You can probably find a mix of new, new-but-vintage-looking, and actual vintage t-shirts there in every category imaginable.
