Rent the tank, cylinder fitting, gauge, balloon filling nozzle, and safety equipment. You’ll never use the equipment again. There are numerous vendors with enough information online so that you can figure out the tank size that you’ll need.
Note that a full cylinder of helium may have a pressure of 1,800 pounds per square inch gauge (psig). While helium is inert, the cylinder must still be handled with care. (Arguable, there are potential oxygen-deficiency issues, but helium rises [compared to carbon dioxide] so the risk is much lower in a large room.) You’ll need either a cylinder stand or a way to secure the cylinder upright against a wall. And you’ll need to store it in a well ventilated room. You may need a handtruck to move the cylinder around, too. (If a cylinder falls over and happens to hit something that damages or cuts the nozzle, you’ll have a torpedo careening around the room or across the stage.)
I found this link via the OSHA website as a good example for safe handling of cylinders.
http://www.sunysb.edu/facilities/ehs/occupational/cg.shtmlBe aware of the jargon of the gas industry. A gas salesman saying “Cubic feet” usually means “standard cubic feet” which for all practical purposes refers to the actual volume of the gas in the balloons. The actual volume of the cylinder will be a small fraction of the “cubic feet” of gas that you will get out of the cylinder.
(The other “hazard” will be those who will suck out the contents of a balloon to make their voices high pitched and squeaky…)
Joe