Best and cheapest program is go to your local O2 plant like NORCO in our area. The smaller steel bottles have pretty much been replaced by aluminum or fiberglass wrapped. The US sizes of those two will not fit in the ASW 27 or the ASG 29 spaces and most places will not refill the European bottles because they are not DOT approved. At the plant, there is usually a bottle grave yard, ask if they have any D size steel bottles. Over the years I was able to obtain 12 bottles, some for me and some for others. Arranged to get hydro tests, new valves and fills for about $45 each. Not sure you can get the same deal but bet you can do better than the $150 plus I have seen them listed for. You may have to do some fine sanding around the hole behind your seat and I removed the padding from the cup at the back of the hole to increase the space of the valve and handle from my seat back.
Tom Boise, ID