Kits must be available because my local brake specialist did mine for the HX last year. They fitted all new silicone fluid compatible seals and a new pressure switch. The body was all blast cleaned and came back looking brand new.
If you can't find a place locally, give 'Perth Brake Parts' a ring.
Cheers

Most brake places wont have the knowledge on a HJ any more.
I bought two PBR branded repair kits about 3-4 years ago.
The big seal on the back of the prop / brake fail module, failed and all the fluid seeped out while I had the car off the road, was a bit shocked to have no pedal when I got in to move it.

Keep trying older mechanics, brake and spares places they may have one still sitting around try there is a place in Mitcham Victoria that specialises in discontinued stock you could try. I cant remember the name though. If you find an old kit one make sure the rubber is still OK the one I got were in a sealed plastic bag and seemed OK the cars brakes work and it doesn't leak)

I have copied the holden section of the online PBR catalogue and couldn't find it I suspect it is a part used on other cars as well and was not listed as a specific holden part.
The newer places wont have them. I thought I kept the packet with the part no, but I cant find it sorry.

PBR have stopped making Master Cylinder kits as well, you can buy new master cylinders though, brake places must now have a monopoly on the parts.
They are probably a common part so if you knew the numbers you could probably get the individual seals.

Good luck