Hi, and welcome!
You used to be able to but Underworld iron-on transfer packets to make your own shirt, as well as a few assorted shirts with "A Hundred Days Off" designs from underworld.printingpete. But that site seems to be no more (at least, i can't find a functioning link!)
As for shirts, i've looked a lot online in the past and never found any shirts for sale. So, looks like you need to either (a) go to a concert and get one! (b) scour eBay or ask nicely around here if someone will sell you one of theirs! or (c) make your own! I've made a number of Underworld shirts in the past for very lil money/effort, thanks to iron-on printer sheets and a computer. i'm modelling one here
:
http://www.myspace.com/geopainter
holden