They are anesthetized first, then asphyxiated. (First they sleep, then they die painlessly).
Depending on how many U want to kill, you shouldn't need to do a large tank. CO2 is heavier than air, too so it will be at the bottom of the container down by the rats.
You can have a valve an knob installed where you have the canisters filled. Or any welding supplies / refiller shop has them. That's were I got mine. Canister filled, valve w/on-off and the hose only cost me $46.
Or check out the Feeders forum. Brad and Bryan and a few others that do feeders to sell, have some great plans and pics for HUMANE ways to gas them for killing. Check the sticky @ the top of the forum main page.
RuLyn