OK, lets start here with the safety aspect. CO2 is dangerous stuff (anything under pressure is really).
Any tank that buy outside the country is likely not going to be able to be tested and/or filled here.
I have toyed around with the idea of using some of my old paintball equipment (co2 tanks, ASA's, macros lines etc) to fit a co2 sprayer onto the intercooler.
Old paintball markers run on food grade CO2, can get big bottles of it from places like BOC Gasses. You fill by weight and need a simple fill station (can buy from a paintball equipment distributor).
The good thing about CO2 is that it is self regulating to ~800psi, you could utilise a regulator designed for paintball to drop pressure if needed.
Lastly, don't forget you will be releasing CO2 into the atmosphere so don't have it exhaust near your air filter or any vents.