On Monday, June 4, 2018 at 5:58:37 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Every welding O2 supply house I have been to exchange the cylinder, not re-fill it. It is just so much faster for a welder that needs to get back on the job. This exchange would be only for standard sizes. This may be fine for your glider but in my particular case (Schleicher) I haven't found a welding supply house yet that has the correct (exchange) size for my sleeve.. Most glider operations out west can refill for you. You might try medical supply houses.

I was meaning a standard cylinder that you then transfill into the glider cylinder. Yes the OP may not have an easy or safe way to carry the cylinder that, but that's how some folks without other options do this. No I would not carry a cylinder around in a car.

Many gas suppliers will fill your own non-rental cylinders as long as the inspection is current and they are properly labeled. Some will hydrotesr/recertify out of date cylinders and then fill them. Anything labeled medical or aviators breathing oxygen increases chances it won't get filled as regular industrial oxygen.

The discussion above already covered why you don't say the word "medical".