Oxygen Cylinders are available in a variety of sizes as well as options for post tanks or toggle tank (able to open and close the cylinder with your hand rather than using a wrench). All cylinders are shipped empty to be filled by your local oxygen supply company. Some companies will only fill their own tanks, so it's always a good idea to check with them first.
M9 or C cylinders hold 255L of oxygen when filled.
MD or D cylinders hold 425L of oxygen when filled.