There is no easy way to administer CO that would be regulated and safe. Plenty of people use CO2 with great success, do some searching around and you'll find what you need if that's the avenue you pursue. Dry ice, vinegar+baking soda, or paintball-style canisters with regulators can all be used to pump CO2 into a kill box.

My euthanasia of choice is cervical dislocation. It is super easy, quicker than any other method, and free.