Keep in mind that you only need to chill the bottom third of the corny, as they dispense from the bottom, so a big igloo round cooler will work fine.
A local HBS here has sorted out a way to use Paintball gun tanks (About the size of your standard Propane Torch bottle) with a standard regulator for portability. I have bought one of these for "emergency" use in case my gas bottle runs empty and I can't git it filled immediately. He also refills them for $5 each.
This Isn't Nam Smokey, There Are Rules.