well depending how big your grow op is
if your a small grower like 400watter or 600 watter you can just get your self a paintball co2 tank preferably a 20 oz w/ on/off valve and a macro line to run into your intake
and if your a big grower ask any local restaurant or place they have fountain drinks
they always got empties in the back mostly 20lbs throw some cash and there yeah go!
rent em, lease em, buy em. 20 pounders, 50 pounders. Welding shops got em, Fire extinguisher dealers got em, A big Company with lots of stores everywhere in USA called Praxair has a whole lotta em. Prax refills em too, cheap like $12 where I am at.