There are certainly companies offering hydrotest services.

If you pressure a vessel with air and it fails, the huge stored energy will mean that pieces fly all over and cause a real mess.
Since water compresses very little [not zero, but about 2% at RT and 1000 atm] there is very little stored energy, so a failure is much less messy. Also, leakage of very little water will drop the pressure to zero.
However, you NEVER do a hydrotest with out protection in case the vessel fails !!! So the vessel is put in a bunker while the techs stay outside and run the pump and read the gages.

Try Autoclave Engineers [Erie PA], EPSI [Andover Mass], National Forge [Erie PA], LEECO [an awful name for a pressure vessel company !] and HIPco [Erie Pa] if you have a big [5 gallons and up] and high pressure [10,000 psi and up] vessel.

If you have a smaller lower pressure vessel, most cylinder shops can do it. CHeck the yellow pages for your area. You can also call gas vendors, since they have to test their cylinders periodically. Google also works