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In regards to this CO2 Tank discussion there are CO2 bottles available on eBay at the moment from a number of sellers if you're looking for 2.6kg & 6kg CO2 cylinders.
If you were to purchase second hand cylinders, you will have to make sure they are definitely Australian standard gas bottles with our Type 30 connector (there are some US ones that most fillers won't swap/fill). And within the required hydrostatic free testing period. Ranging from 5-10 years (depending on the gas bottle) of when the bottle has been stamped. If you have bought a much older gas bottle, gas suppliers or commonly fire stations will be able to hydrostatically test and stick a sticker for testing on the bottle for you for a small fee, giving you another 5-10 years of piece of mind.
I'd personally suggest having a back up bottle for those who dispense regularly. I guess my mindset goes the same with an LPG gas bottle for your BBQ, running out half way will always cause a stir with everyone involved! Having a backup means less of a headache at the end of the day