"The Scuba Diver Store" in Milwaukee, WI assembled an adapter for me. CGA 540 nipple (part Global 4521 1oxC) to Scuba port Converter Block (part Global 4502 1ox). Cost including shipping was $83.15.
Today my local tech diving store (Virginia Scuba) filled the tank with Oxygen for $0.50 per cubic foot, which was $7.50. That compares favorably to $50 at my local airport, and $35 at the local industrial gas place (which takes weeks to do a fill in any case). The adapter will pay for itself on the second fill.
The adapter, as delivered, leaked. The diving store added more teflon tape, problem solved.