I went to one of those hardware store chains, Leroy Merlin. Similar chains in Italy are Bricofer, Obi and others. There you have to look for the soldering sector. You will find there cylinders with oxygen, or CO2, and also argon and argon-CO2 mix.
There are let's say three kinds of cylinders that you can buy.
The large and serious cylinder, heavy to transport, which is used e.g. by people kegging the beer, or for real work in apartment restructuring etc. Those are refillable cylinders. They are filled at a very high pressure and have specific "valves" (pression reducers).
This is an example: Bombola Co2 da 4Kg Nuova ForHome® Ricaricabile Con Valvola Residuale Certificata
They might be overkill if the purpose is only capping.
Then you have the smaller, non-refillable cylinders. In the case of Oxygen, they come in two formats: small, around 24 litres of expanded gas, they are very light when you take them in your hand they seem empty, and "larger", not in physical dimensions but in weight and capacity, they hold more than 200 litres of expanded gas, the cylinder itself weights 1200g. Both kinds use the same pression reducers I do believe.
The pression reducer is often termed simply "for non-refillable cylinders", or it is termed "M10x1"
This is one example, a "heavier" CO2 non-refillable cylinder: Bombola a gas co2 AWELCO 0.95 L. Prezzo online | Leroy Merlin
The connector is called "filetto europeo", European thread, because that's a standard for those kind of cylinders. Basically M10x1, European thread or "for non-refillable cylinders" will get you the right pression reducer.
Note, though, that oxygen cylinders (which can be used for wort oxygenation, with a 0,5 micron aeration stone) need a specific pressure reducer, which is marked as "oxygen".
Actually now that I think about it, M10x1 is the one for oxygen, and generic "for non refillable cylinders" is for generic gas. Maybe (I did not test it) the two threads are different. [EDIT: tested, the thread is the same]
In any case, the assistant will give you the right pression reducer for the CO2 cylinder and the right pression reducer for the O2 cylinder. These cylinders typically are not sent by courier for the danger of explosion when left in hot temperatures, so you'll have to go to the hardware store.
Examples: Riduttore di pressione. Prezzo online | Leroy Merlin
The gas gets out through a "hose connector", you can couple there, typically, a hose with 3mm internal diameter. I bought this on Amazon.
A silicon hose is very good because you can immerse it with the aeration stone in a vessel with water and sanitize, or even sterilize it with a microwave oven without having to touch the stone to fit the hose again, you just sanitize all together.
Oxygen is easily inflammable and CO2 can suffocate you and actually kill you, so this stuff is to be kept with some caution. I keep the cylinders outside but NOT in direct sunlight, that creates a danger of explosion.
In order to use them, you just close the tap on the reducer, fill the reducer on the cylinder, and then open the tap for the gas to get out. No particular manoeuvers.
The cylinder does not stand right by itself so I will use a bottle-holder to keep it up. One hand manovrates the gas tap, the other keeps the silicon hose inside the bottle. I will report here how it worked.
Just as we are there, this is an oxygen cylinder:
Bombola ossigeno per saldara ossido acetilenica a gettare Codice originale marchio:UNIVERSALE (OXYGEN) Note descrittive:Cartuccia di Ossigeno per Turbo Set 90 Capacità 1lt - - Ennebiservice
That's, again, more than 200 litres of oxygen. If you oxygenate beer (at pitching stage) with 1 minute at 1 litre/minute of flow, you have more than 200 batches of beer for this cost (plus the cost of the pressure reducer).
Oxygen for "soldering" is basically identical to the one used for medical purposes, for our goals. You will find very pure gases in there, no oils or other unwanted stuff.
Another option is to use the SodaStream cylinders with an adapter, as in this video:
"SodaStream" cylinders are actually generic gas cylinders of a standard which is called "S30", so you need an S30 CO2 cylinder, not necessarily a SodaStream.