We recently finished our own shirt cannon bot, and (like other posters have said) I'd recommend trying to get your hands on a SCUBA tank. We asked a local dive shop if they were willing to donate on (it's a good cause) and they gave us an old one AND a first-stage regulator, too. The capacity is enormous; I'd estimate about 80-100 shots at our usual 70 psi. The only hassle is getting it refilled, but we know in advance when it needs to make an appearance and can plan accordingly.
In addition, you're definitely going to want to grab an air-rated valve. We found
a good one for <$25 on amazon (even has free shipping) that is rated for well beyond any pressure you're going to shooting with, and is also very quick to open and close.
Looking pretty great so far though, the turret mechanism and tracks are going to be a hit when you unveil it.