You would need a prescription to get the medical O2 cylinder & regulator. The same for refilling a medical cylinder. The welding type cylinder would be easier to come by and less hassle to fill or exchange. I'm not sure it it's even legal to buy or sell the used medical type equipment.
You may have to bite the bullet on the SS diffusion stone and just buy it from one of the home brew suppliers online or maybe your LHBS has them in stock. You could use aquarium type air stones, but I doubt they would withstand boiling temps. I would just get the cheap ones and use a new one each time. I think they will handle sanitizing with iodophor or starsan, but I have not personally tried that as I use an SS wand type thing.
I don't know what the pressure is for the small, disposable, Bernzomatic type O2 cylinders is, but I know it's not thousands of psi. More like a hundred or so. Those small cylinders only contain about 1 cubic foot of Oxygen (at atmospheric pressure). By comparison, the small welding type O2 cylinder I use holds 20 cubic feet.
It's possible that you could use a needle valve to control the flow from a disposable cylinder with reasonable precision, but you would still be faced with the odd thread match up. The bottom line is that it's more trouble than it would be worth with no guarantee that it would work well.
I do realize that home brewers are notoriously stingy, or frugal, if you prefer, but sometimes it's just best to spring for the right equipment. It's usually a one time purchase kind of thing, so the cost is not all that much in the big picture. If you are wincing at spending $50 on this hobby, beware that there may be rough water ahead. Remember, these are toys for your hobby and any expense can easily be rationalized. I do it all the time.
