Here is a CO2 bottle that is made for what you want it for. You can run air tools off this baby. High priced but here's a link:
PS I apoligize, it's a Jeep catering site.
Quadratec-powertanks
First, I realize that Bretwalda already posted the specs for these, but why would you want to half ass something that could kill you very easily? Powertanks are made for off roading use. They have the proper regulators ect. No questions asked when refilling, all safty documentation...
Second, I would not mount a pressurized tank outside the vehicle, an accident or a rollover could get complicated quickly <img src="/forums/images/graemlins/zombie.gif" alt="" /> <img src="/forums/images/graemlins/zombie.gif" alt="" /> <img src="/forums/images/graemlins/zombie.gif" alt="" />
It's kinda like saying you want to bolt an aux fuel tank to your rack. <img src="/forums/images/graemlins/scared.gif" alt="" />