well some places in the US may ship rebuild kits to you direct to SK if you call them, also have you talked to NAPA in person about it yet, because they maybe able to help out more than their online webpage can do.
also i would call around to brake shops in the major cities too, they maybe able to help you out over the phone.
i haven't rebuilt my booster yet, it's not original to my truck so it's a future project, so far it stops well without a booster, but they are only 1 tons
there is a 2 ton and a 2.5 ton on my farm that both have their original boosters, i doubt they are operational.
i seem to recall the last time we used one for a water tank, the brakes consisted of a piece of wood with 8 ft of bailer twine so you could pull it out from the wheel and reel it into the cab from the driver seat
-s
