A Boss PSA-style PSU should work fine.
All of our pedals use a standard 9v DC adapter with a negative ground
Well, 9v DC, centre negative is the Boss-style PSU spec, the only one that in the pedal world that can be called 'standard'... but what about that 'with a negative ground' bit?
I stand to be corrected, but I don't believe that a typical pedal power supply, including the BOSS PSA, itself presents a 'true' ground to the pedal. 'ground' in a pedal context usually means what side of the power connector is connected to the pedal's case (and, consequently, the signal cables' shield).
Usually with pedals, this is the 'negative' side - i.e. the centre. I'll quote http://www.diago.co.uk/adaptors/positive-ground.html :
The Diago Positive Ground Adaptor allows positive-ground pedals to be
powered alongside regular negative-ground pedals via the same power
And I guess this is what JHS are saying about their pedals when they say 'with a negative ground' - just that they're normal in this respect. I guess it also means that if you are using a PSU with a true ground, it would have to be negative ground.
The only time I've had to worry about what is 'ground' was when powering pedals off a daisy chain - in this case, you need to know which side (+ or -) of the PSU input is grounded to the case as if you mix up 'positive ground' and 'negative ground' pedals, you'll short your PSU as both + and - will be connected to the case.