[Delphinus]

I've been using the Recharge Centre for my refills and they occasionally have used cylinders for sale, although usually only in the 15lb and 20lb sizes as they tend to be refurbished fire extinguishers.

Most fire extinguisher or beverage suppliers can set you up with a refill but it tends to be a few days and usually $30ish for refills (no matter what size cylinder you own).

J&L and Ocean Aquatics sell cylinders in the 5lb and 10lb sizes usually.

The difference between cylinders made for aquarists versus cylinders refurbished from other things (pop dispensers or fire extinguishers) is that the ones for aquarists tend to be aluminum whereas the others tend to be steel. Aluminum is nicer because it looks nicer, doesn't corrode, and the throat valve uses an O-ring to seal. In steel the valve stem is tapered and to seal with they have to grind out a little bit each time they do a hydrostat test. Which means it has a finite lifespan, every hydrostat test edges it close to the end.

The hydrostat test needs to be done every 5 years. Mind you with the steel cylinders, they are good for a good number of tests so even though the lifespan is finite, it's still pretty long. I saw someone using a 60-70 year old cylinder once, it was fugly but it still worked.

Or you could just say "forget all this" and look at renting a cylinder. No worries about hydrostat tests or fills, just exchange the empty for the full and you're done. But then you have to pay a rental fee every month.

(Honestly it's six of one, half dozen of the other if you ask me. Some people prefer renting, some people prefer owning. Each has it's pros and cons so whatever works best for your situation.)

If you prefer to rent you can look at any gas supplier. Oxypro, Medigas, Liquid Air ("AIr Liquide"), BCG, etc. etc. Also check with beverage suppliers, also check welding suppliers.

Regardless whether you rent or own the cylinder you'll need your own regulator. J&L and Ocean Aquatics or any of the mail order supply places with a decent drygoods section can set you up. What you want is a dual guage regulator (inlet and outlet pressure), a needle valve for fine tuning the gas control, and a bubble counter so you can see how fast the flowrate is.