Recently Hot Topic had released some shirts, however, I'm not sure if they are still around.
You may also want to check out
http://whedonesque.com/ which is the weblog for all things Whedon.
I got my 'Joss Is My Master Now' shirt from Think Geek and my 'Sunnydale' shirt was a gift.
I don't do the t-shrits much, but if I see something, I'll send it your way.
