ThinkGeek are good for unix shirts and general geek apparrel (I know they probably compete with OCUK in a very few product lines, but I doubt anyone will mind with them being in a different country and having awful shipping times).
I currently have nice black ones bearing....
"No, I will not fix your computer"
and
"I don't work here"
Next up I think I'll go for
"Fear me, I am root"
and/or
"there are 10 types of people in the world, those who understand binary and those who don't"