I go to the place that hydrotests oxy cylinders for welders and
medical users. I have yet to bring them a bottle that they would not
or could not test.

So, I'd think that the question becomes: Who is imposing the
requirement that the test be done by an "FAA authorized oxygen service
station?" If that verbiage is in the specific documentation or
operating limitations for your sailplane, you might be stuck. But if
not, just get it hydrotested at any reputable pressurized gas vendor
and get on with life.

Thanks, Bob K.