Michigan is home base for a lot of Floricultural companies. You may want to try BFG supply. I've brokered through them for 22 years.

I'm really not understanding why you want to provide C02 to stock you must be storing in winter in a barn. Are you talking dormant stock? Are they lit? Heated?

Plants take in carbon dioxide primarily when they are actively growing and in the presence of sunlight. In the dark they take in oxygen. If you need CO2 supplementation at all, it would be in your greenhouses when the plants are ripping, not in a dark barn in the cold.