Wal-mart. I found a black one and a tan one, both for about $10. My only gripe with the tan is how easy it snags, but they are nice.

As much as people hate wal-mart (and I do too), I do break down and buy some things there, like socks, underwear, and Dri-Star....mostly since socks and underwear anret worth time shopping around, and well I dunno of anywhere else that sells Dri-Star stuff.

By the way, it seems their selection is seasonal....T-shirts in the summer, stretch fabric polos in the fall, and long sleeves in late fall to winter.