It's cheap, but hard to contain, as soon as it hits air it evaporates. We were messing around with some in physics class. I'm pretty sure it's hard to compress too. We just had some in a bottle and were pouring it on stuff and then throwing it agaist the wall and watching it shatter. CO2 I think would be a good bet. When released into the atmosphere it sprays hard. You could probably get everything you need from the paintball industry. It's cheap too, any paintball place will give you a fill for less than $5.
