The Strida Europe website says that people can buy Stridas and accessories online on their official website after 1st April 2009:
strida.nl
I think you can email them asking whether the T-shirts will be available.
info@strida.nl
If they don’t sell the T-shirts, I think you can buy them from Hong Kong. 
[url]strida 5.0 in charlotte, NC - #8 by chainstrainer]