R134A is used in gas powered air soft guns. You might try to give that a shot... I dunno how pure it is but you may want to look into it. Many online airsoft stores carry it and your local airsoft store should have tons of it in stock too.
Green gas for airsoft is expensive.You can get r134a almost anywhere it is one of the few refrigerant that dont need a license to obtain.But its a waste of time as propane is much better for out application, and alot cheaper.
Actually r404 isn't the mother of all refrigerants.All people new to phase change think r404 is going to automatically make their system perform the greatest which is not true.R290 is perfect for starting out with, once you figure out what your doing then you can go buy an expensive refrigerant and use it instead of wasting it on learning refrigeration.
A good refrigerant is good to have, however along with that refrigerant you need everything calibrated and put together right to take advantage of the gas. Start with propane and go from there if you dont like it.