I am looking at one one-way trip (not in my plane
) which will need oxygen. I might take an old ally bottle and leave it out there. I have found that I can buy a brand new one, mail order, for the same (about $300) as it costs to get one checked and pressure tested over here.
AFE at Kidlington got my aviation cylinder recertified , including the refill (if you pick it up - shipping full cylinders is prohibitive) for £50 inc VAT.
I am sure other places can do the same.