Good Luck!? You might want to try ebay or if you're not going to one of the upcoming shows see if a board member that is going might pick a new 2006 shirt up for you.
Btw, they were selling those "retro" 1991 Illusion Tour shirts at Kmart for $2.99 about a month ago.? I also remember some guy in line at the Hammerstein Ballroom giving them away to people.
It is a Kmart version as I have it with the tags still attached, I snatched it off Ebay for a really cheap price cause it was the only XS I could find and even still it's big on me. I am gonna have to start shopping in the GNR childrens section