Just call your local welding gas supply dealers on the phone. (That thing you that takes pictures and can talk to other people with.)
Just rent a liquid Nitrogen dewar for a week or so. Its fairly cheap.
Most welding supply chain stores, at least Praxair does, also carry other cryogenic liquids as well.
You can also get liquid, oxygen, Argon, hydrogen, and helium if you want too!
The commercial liquid oxygen is cheap but the other stuff gets expensive fast!
I would suggest getting the liquid oxygen and hydrogen. Then you really can be Rocketman!