We based our t-shirt cannon off of 254s layout. What we used and thought was safest was the K value that connects to the 3000psi SCUBA tank and then the 150psi SCUBA regulator which is not very expensive.
The accumulating tank at 150psi for us is enough to shoot one t-shirt about two hundred feet.
Our team's page with info about the T-Shirt Cannon. Hope to get an engineering journal of it within the month.
http://kalanirobotics.com/team3008/R...velopment.html
and a video of ours
http://www.youtube.com/watch?v=Vdh5rjAcDMI
Good luck!