Just go to a welding gas supplier and purchase or rent a tank. They come in sizes from about 16 inches tall to around 4 feet. The bigger they are the more O2 they hold. I use one that is about 3 feet tall and it lasts me 2 or 3 months with DAILY us for up to about 30 minutes each time. Costs about $20 exchange the tank for a full one. You will have to purchase an oxygen regulator for welding tanks. It hooks up to a welding hose but you can get a fitting that connects to it and allows you to attach a "food grade" plastic hose to it with a small hose clamp. I then just hook up a standard "blow gun" from Harbor freight to it and set the regulator to a pressure that allows me to stick the blow gun in my mouth and feeds O2 at a pressure/volume that allows me to take a normal deep breath from it. You can then take the tank to the same "Brand" welding gas supplier pretty much anywhere in the US, any time they are open and exchange your tank for a full one. I suggest buying a regulator sized for one of the "mini" Oxy/Acetylene welding tank sets used buy Air conditioning or refrigerator service companies. The same regulator should fit ANY size of welding oxygen tank the suppliers have, and the smallest O2 regulator you can buy will put out more than enough O2 for this purpose.. It works best if you always carry your paperwork from the original place from when and where you purchased or rented your tank. I happen to be a welder, so I actually OWN my tanks and have 2 different sizes. And you don't need to tell the gas supplier that you use the O2 for cluster headaches. In some cases it might help, but in other cases they may refuse to sell you the tank or re-fill it. They will say that you MUST use only "medical" O2 and tanks for that. But the truth is that you can follow one of their delivery trucks to any hospital and watch them unload the very same large, standard, all green heavy steel tanks used for welding at the hospital, where they are then taken to their basement "tank room" where they have a whole bank of those "welding" tanks hooked up to feed the entire hospital with "MEDICAL" O2. (What a joke!) By the way, I use NEXAIR for mine. It's the same company I bought my tanks from about 40 years ago, but they have changed names several times.