A Boss PSA-style PSU should work fine.

https://www.jhspedals.com/faqs/ says:

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 supply.

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 'ground' is when powering pedals off a daisy chain - in this case, you need to know which pin 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.

A Boss PSA-style PSU should work fine.

https://www.jhspedals.com/faqs/ says:

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 supply.

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.