Oxireyes, in Riberas, delivers medical oxygen 24/7. 33 3104 5218.
The way it works is you can rent a tank from them (the big 5 foot tall tanks) for 5,000 pesos, I think. When it gets low, you call them and they´ll come and switch it out for a full one. Refills cost 1,500 pesos. They will also fill your own tank if you have it, but it might take half a day before you get it back if they do the pickup/delivery. You could also take your tank to their place in Riberas to get filled.
You can also rent oxygen concentrators that go up to 5 L/m at Lidia Zamudio´s medical supply store (West Ajijic 376 688-3385, San Antonio 376 766-2088.) I know Lidia also used to rent out small portable O2 tanks, but they may be in limited supply since COVID.