I saw an ad for Polo stuff for dogs in a magazine last November/December 2003. It was available online at
http://www.polo.com up until Christmas or so. After that, they pulled it. I guess it was a Christmas promotion only and it was Yoda's Christmas gift. I have a matching one in my size and we both wore it on Christmas Day.
Hopefully the promotion will be back this holiday season. You might be able to find some Polo shirts on ebay now. That's the only place I can think of that might have it. Usually Yoda gets stuff from Petco, the Small sizes usually fit, sometimes we need to alter things a bit. They are on sale a lot as well.