I have a can ...... I think 15 oz.... sitting on a shelf here in the house... easy to get from Ebay....
You are completely misrepresenting the effectiveness of soap and water... that is totally unrealistic for finding a leak in the evaporator.... or many places on the car where insulation covers the joints.... or mid hose.... as was other leak on my wagon... that long line which goes up over the engine leaked...and the evaporator leaked....
I think I paid $130 for my nitrogen tank... I bought it because I do not like to pay demurrage ,,, I do not use my gases very fast... might take years to empty one..
and I have a plasma cutter.. so in addition to testing ac systems.... I can use it for that....
How a Plasma Cutter Works
https://en.wikipedia.org/wiki/Demurrage
'''Gas cylinders
The rental fee for a gas cylinder assessed by the vendor until the tank is returned.'''
I am about as frugal and practical as anyone you will run across... and I understand the physics..... and I hate to do things twice when a little extra work can make the first time a slam dunk.