I'm no authority but I considered the CO2 and N2O cartridges that you get from lots of places for a lot of uses. They're often sold as instant cycle tyre inflators (CO2/N2O), soft drink carbonation (CO2) and a whole host of other uses. Try googling for 16g CO2 cartridge, that should get you started.
The cartridges have liquid gas in them which is a big bonus when it comes to quantities (even the small cartridges have a surprising amount of gas in) but a big issue when it comes to decompression, since they tend to freeze if you dispense the gas too quickly.
To give you an idea of gas consumption (approximate numbers to give ball-parks):
CO2 liquid density is very close to that of water, at 20 deg C it's 1032kg/m3.
A 16g cartridge will hold about 15ml of compressed CO2 but that's at over 19 bar. You need two bar of pressure to overcome the outside water pressure at 10m depth so each cartridge can dispense 9.5 * 15ml or 140ml of gas at that depth.
At 2m depth you need 1.2 bar of pressure to overcome the water pressure which gives about 240ml of gas.
Depending on your sub characteristics (i.e. are you running a small sub?) this may or may not be enough gas. You can get much larger versions of these cartridges and if you have bigger subs you can use CO2 cartridges from welding stores. The refill cost on these is not low but it's not expensive either.
And, of course, if you can use some kind of air recycling method, e.g. snort (use the gas to get you to periscope depth and then snorkeled air to fill the MBT) or RCABS (use an air or water pump to inflate bladders/cylinders with water/air to change density) then your gas capacity only needs to be enough to vary your density and depth, consumption here would be very low.
Personally I don't like gas consumption systems, they scare me with the possibilities of out-of-gas errors, and I much prefer recycling systems. Using gas an emergency MBT purge, though, would be very useful and I've considered, but never implemented, this. It would be easy to implement as part of a fail-safe system, trigger it on loss of transmission signal and open a solenoid into the MBT from the gas cartridge. In these kind of emergency situations you wouldn't necessarily care about the freezing effect of the gas expansion although you would need to make sure the electronics were sealed to avoid water condensation when the air is cooled.