The subtly named guyabera.com has some.
Let me suggest an alternative, too.
TravelSmith, Eddie Bauer, and Land's End all sell shirts that look just like traditional sports shirts, but with straight hems designed to be worn out. They're what a lot of the men in their 20s wear with a t-shirt underneath. I started a thread over General, I think, about these. I'll have to check in over there and see how it's going.
