welding tank

What I did is I found a 15lb co2 tank at a canteen service that was out of hydro. Gave them $10 then took it to a welding supply and paid for the hydro....$18 then I was able to get an exchange for a full tank for around $17 more.
after a few refills they quit handling 15lb tanks so they upgraded me to 20lb for just the price of the gas. A bit over $1 a pound. Now I always have a fresh tank that is always in hydro. Go with a welding supply get a tank and do the exchange bit. You do have to make an upside down mount....no dip tubes on exchange welding tanks. But it only takes a min. to carry a tank in and get a new filled one, no waiting. john