Call your local welding supply house. Places like TriGas, AirGas, etc.
will have someone local they use or can probably get you to someone who
knows.

John Smith wrote:
Scuba tanks are usually filled to 3,000 psi and rated to 4,000 psi, so
they will know how it's done.

But few if any scuba shops can test themselves, they usually send the
bottles to a test facility. Which means that it won't be much faster
than sending them directly to MH.