I have one of the smaller cylinders, but for the right price I wouldn't mind picking up a larger one so the gas lasts longer. Would also be interested in a used regulator otherwise I will be buying a new one from HF tomorrow.
I traded my old one in for a larger one at my welding supplier. I own my tank, just had to pay the cost difference in the tank size. Might be one option, if your supplier offers this.
I bought a 20 CF tank from harbor freight that I have been exchanging.... thinking I would like a 60 or 80 CF tank... I am a little scared of what they might charge for that though.