"You might try medical supply houses."

Medical supply houses are a BAD idea. First, they will most likely demand a prescription from a doctor for refills, as medical O2 is regulated by the FDA. It will also be much more expensive. Second, virtually all medical O2 cylinders use a "post" type (Puritan-Bennett CGA-870) fitting. Aviators Breathing Oxygen and welding oxygen cylinders use the CGA-540 fitting. (European cylinders typically use the DIN-477.)

If you have your own transfiller, you might be able to persuade a welding shop to give you a fill, but I suggest cleaning all the fittings on both cylinders and the transfiller with Formula 409 or equivalent. (No acetone, alcohol or other petroleum based solvents, for obvious reasons.)