[helldritch]

The typical way to produce small scale liquid oxygen (LOX) requires liquid nitrogen (you use the liquid nitrogen to cool the gas as it passes through some transport medium like a copper pipe and collect the cooled liquid).

Purchasing LOX isn't too difficult, there are many vendors willing to supply large amounts of it, and a few vendors willing to supply small (experimental) amounts of it.

Storing LOX is the real challenge, you need to keep it at ~310f (~-190c) which requires a well insulated cryogenic storage chamber. Typical industrial sized storage tanks have a few metres of insulation to keep the heat out, and typical small-scale tanks have around 1-2 feet of insulation.

You're probably looking at a viable minimum storage container size of between 500-1000 litres (any smaller and your insulation layer will be so thin that you'll lose 10-20% of your stored oxygen as gases per day), with the typical minimal size being in the 2,500 litres range (which normally lose 1-2% of stored oxygen to gas per day)

You can buy very small containers (2l, 5l, 25l, etc, these are for nitrogen but you can get them for oxygen too): https://uk.vwr.com/store/product/2104367/liquid-nitrogen-dew... but these lose the liquid very quickly and you would likely want them delivered the same day you need them, resulting in a just-in-time delivery failure point.