You know, (as mentioned) you can buy a bulk c02 tank for pretty cheap from a paintball store, ranging from a 9oz to a 20oz. Buy them with on/off valves, and you've almost got your own mass euthanasia center! Just get a pop bottle (2L), or large water jug, and cut a square flap on the back big enough to fit mice. Cut some paper to just larger then the flap size, and glue / tape it to the piece of bottle you just cut. Tape one side of this 'door' over the flap, then insert the C02 tank nozzle into the neck of the pop bottle. Twist the on/off valve, and turn off when the bottle is suffitiently filled (Making sure to tape down the flap to prevent precious gas from escaping, as well as re-sealing the jug/bottle). Tada! C02 tanks are pretty cheap, just don't buy from walmart of canadian tire (if you have one); find a good, reputable shop for the best prices, and you can have them filled for cheap at most scuba shops, or paintball shops.
Hope that helped, sorry if its a pre-existing idea.