VI Reptiles,
Check out this link on building your own CO2 chamber:
Click here
As for throwing rats in the freezer to kill them, that just sounds lazy to me. I can't wack a rat to feed my snake either (just a softy here), but I'm a bit more humane then freezing the stupid thing to death. That is just not cool. I built this CO2 chamber, and it works off of vinegar and baking soda! HAHA! I love chemistry!