Something along the lines of nitrous, however would not be used in the same manner. It just needs to be a very cold gas that can be released into the atmosphere. Also I need it to be fairly inexpensive and with decent availability. I wan tot use it as a pressurized cooling agent.
How so ? The proper choice of coolant depends on what you intend to do. Liquid nitrogen is cheap and easy to get in large cities. It readily turns into a very cold gas. Dry ice + acetone (or isopropanol) can be had nearly anywhere, but falls a bit short on your requirement that the coolant be a gas. Freons are another possibility. If you don’t want to specify further, here’s a whole pile of information on Gases and Refrigerants.
Any gas is cold if you lower it’s temperature enough. Do you want a gas with a high Joules-Thompson coeficient? (The higher the JT coeficient the colder the gas gets as it expands. Freon is used in refrigerators because of this.) Or are you simply ineterested in getting a cold gas no matter what the energy cost of cooling it. In this case, choose anything you want. Helium will get the coldest, but good luck obtaining it.
For that matter, any thing is cold if you lower its temperature enough.
Some people think liquid nitrogen is “cold.” This is incorrect; the temperature of liquid nitrogen depends on its environment. As an example, we have lots of LN[sub]2[/sub] containers where I work. The LN[sub]2[/sub] inside the container is not cold! It’s at room temperature and under high pressure. Only when you vent it and allow it to boil does the immediate surrounding environment become cold.