You can't buy parts for any brand of control valve from anyone but a water treatment dealer for that brand of control valve.
You also can't buy a part of a stack, you buy a stack. Also, you should never replace just a stack, or Fleck seals spacers, or just a piston, without buying a new piston, or stack, too. They wear on each other, and usually you can't see the wear with the naked eye.
Yes I can sell you any parts you need for a stock Clack WS-1. And I tell you how to do whatever to replace them properly. You have to call me for pricing.
What is this "cap for the brine line" thing?