Go to a local commercial gas company and state you need a co2 tanks because your job requires that as a too being you are and HVAC/R tech. CO2 is used in that trade to blow out drain lines, blow out condenser coils, etc... they will make you pay a deposit on the tank around $60.00 then you will need a regulator and then a hose if prefered also you will need to pay under $30.00 for the tank to be filled. About $200.00 total but is it worth it?? research and you can find other ways of making CO2.
Depending on how big your set up is, and if it isn't too big you might want to scrap the Co2 tank idea and just make a DIY Co2 system. Get a 1 gallon water container, 1 packet of champagne yeast, 1 cup of sugar, some epoxy, plastic tubing, and a half-liter bottle of water. Cut a hole into the top cap of the 1 gallon of water jug and run the tubing down sealing it with epoxy to prevent the Co2 from leaking out. Cut the half liter water bottle in half and secure the tubing to the bottom of the bottle with the epoxy or a weight. Add water to the half bottle of water, fill about half way, and fill up the 1 gallon jug with water. (Use Warm Water) Add the yeast + 1 cup of regular sugar to the 1 gallon jug and seal with the cap. In about a couple of hours you will notice air bubbling from the half liter bottle of water coming out. This is a very efficient and cost effective way to generate Co2. The important thing is that you keep the whole system air tight, if not the yeast will not consume the sugar creating alcohol and Co2.