Call around, see if you got someplace like tech-air, or call your local auto parts store and ask them for a place that refills
tanks for welders.

Call them and if they ask, say you want it for your keg party.

A 20 lb tank will set ya back about $140 for the tank and another $23.00 for the gas.

Used with HID lighting, it will do more for your grow than anything else that you can do.

Also, I believe you can rent a really large tank, a 5 footer, for a year, for about $50.00, but you need to set up an account for this option.
Peace

Those aquairium co2 units wont work for this kind of grow.Those make co2 and inject it into the water which is not what you want for ganja.Youd have to figure out how it makes co2 then modify it to inject into the air without the use of water.I actually have some co2 tanks Id be willing to part with. I just grabbed 3 more 20 lbs tanks as I got a deal on all 3.I have a 10 lb tank too.Id sell the 10 lb tank for $85 plus shipping in the US only and Id sell each of the 2 20 lbs tank for $110 plus shipping.Youd be on your own for getting a release solenoid or youd have to manually release the co2.Anyone wants an empty tank pm me great for welders.