When I rebuilt my S2A/S3 caliper, I used an NOS kit but also purchased a stainless caliper piston from HVC Cycle to complete the necessary parts, and of course new pads. I also had a spare caliper that I disassembled and used a slightly better condition part or two off of.
You didn't specify which model Triple, but here's some sources for the 72-75 H1/72-75 H2:
http://www.hvccycle.net/kawasaki-triple-h1-h2-front-brake-caliper-seal-kit-hvc32-1152/
Here's one source located in CAN: http://www.diablocycle.com/Kawasaki-H2-and-H1-Caliper-Rebuild-Kit.html
It appears some kits may not contain all of the necessary parts, so you may have to locate a small rubber part or two to complete the available kits. I'm sure the two sources above also have anything left out of their kits.
Piston:
http://www.hvccycle.net/kawasaki-triple-h1-h2-brake-caliper-piston/
Also consider contacting Jess at http://www.jessbikes.com/. He can usually hook you up with many of the parts you will need for your bike. He may not show all of his parts on his website, so please do email him.
