It will probably come as no surprise that I know a bit about this stuff!
In my experience, getting O2 refills around the countryside varies from difficult to impossible, and is always expensive.
I can't get a refill in Townsville at all and I can't get one in Cairns unless I supply an adaptor that fits my cylinder.
My answer to that problem was to set up my own refilling station at home. With fixed clyinders in the aeroplane you would need to fit out your own trolley.
One cylinder to refill from does not work well as once the pressure in your main cyclinder equals the pressure in your aeroplane cylinder, you can't get anymore O2 into your aeroplane cylinder. You need to have at least two cylinders - which is why you see O2 refill carts with a bank of maybe 4 cylinders.
You always start your refill from the lowest content (ie pressure) cylinder, then top up to your final fill pressure from the second one. When you can't get anymore useful pressure out of Cylinder #1 you get a new one and rotate your cylinders one place to the left! ie the fullest cylinder is used last to bring your aeroplane cylinder up to final working pressure.
You can buy an adapter for filling onboard O2 systems.
Cost me a couple of grand to get set up, but refills cost me next to nothing - compared with about $90 for the last one I paid for on an aerodrome.
I can generally go out and back to most places I want to go with myself and Mrs Dr on O2.
I use Aviation O2 but you can use welders O2 - its the same stuff! But it is hard to get the same pressure (ie fill) with welders O2 cause the cylinders are filled to a lower pressure.
Aviation oxygen cylinders from BOC are at Extra High Pressure = 25,000 kPa.
Dr 