CO is available. We buy bottles which are used to calibrate our air monitors at work.
But I would certainly not use it for fumigation purposes. It is far too dangerous. And without proper detection equipment, you will never know it's there.
Every welding store sells bottles of Nitrogen, and CO2. They would smother anything in your building, and be far less lethal to you as you ventilate the place.
CO is so dangerous because hemoglobin has a greater affinity for CO than it does for O2. A very minute amount of CO in the air will accumulate and concentrate in your blood until the hemoglobin is not capable of carrying any Oxygen, even in an Oxygen rich atmosphere.
If you really want to fumigate the building, it is better to use an actual fumigant, and probably a licensed applicator.
Have you considered what you will do about the aroma of decomposing rodents in the walls and other inaccessable locations?
In your situation, I would go down to D&B or Tractor Supply and buy a 10 lb tub of Tomcat Rodenticide, along with four or five outside bait stations. Proper outside bait stations will prevent anything other than rodents accessing the poison baits. The outside locations will thin out the mouse or rat population before they make it into your building.
Then you have the option of baiting inside the building, but again, there's the issue of rotting carcasses.
I prefer to trap inside. There are many good, inexpensive traps on the market. I am very fond of the Tin Cat or its many clones. Once you catch the first mouse with a Tin Cat, the other mice will smell his urine and feces in the trap, and you will catch many more very quickly.
