I have had good luck with Medical and welding shops getting cylinders
tested. One facility in Albuquerque (Aidant Fire Extinguishers) did
several for us when I was helping run the Sandia Classic Hang Gliding
contest. They charged about $30 and had a 48 hour turnaround.

The only thing they mentioned was that the fiber -wrapped cylinders
('glass, Kevlar and Carbon) have a 15 year service life. Unless that
has changed, owners of these cylinders might have a problem in a few
years. Some fiber wrapped cylinders have been around nearly that long.

I don't think metal (steel or aluminum) cylinders have a life limit as
long as they continue to pass the pressure test and corrosion
inspection.

Mark628CA