Any industrial gas or medical gas outfit can do it. Airco is our welding gas outfit locally. They do hydrostatic testing for most cylinders in the area.
Ask a fireman where they get their SCBA tanks tested. Drop in to a medical supply place and ask them. Any big construction outfit will know.
Lots of scuba shops in AZ. They fill hundreds of tanks a day. They will know where to get a hydro test locally.