Well, that is a hard question but to find T-shirts or hoodies that are dedicated to mountain biking sites that is harder.
I found these unusual garments at:
http://www.slickactivewear.co.uk/shop/index.php?cPath=10&osCsid=d0575de566c0e1a2826af57d5c015ea6
Good stuff I think. Hope you find it interesting too.