I can still get the little red oxygen bottles, but the cost is nasty. I wound up buying a oxygen regulator and 20 cubic foot tank. The cost of oxygen is way cheaper, but the up front cost is not good. Each refill of of the 20 cubic foot cost $30 and last 3-4 years. I'm pretty generous with the oxygen, so those little tanks were costing me 30-40 bucks a year.
There's is no problem getting the tanks refilled.