The final Fleck 7000 was produced in September 2016. As you purchased your system in 2018, if obtained from a reputable local dealer, I would expect to have been advised that the valve is from remaining stock as it is no longer being produced, thereby providing you an option of choosing to equip your system with an alternate, more current valve.
Although parts for discontinued models will typically remain available directly from Fleck, usually for up to a decade after production ceases, when discontinuation of a control valve is initially announced, distributors and dealers will often stock-pile the most common parts, thereby depleting the manufacture's stock more rapidly than anticipated.
Clack long ago made the policy decision to stop selling to on-line dealers which focus on the DIY market, instead making their products available to water treatment professionals which directly install and service the systems they sell. Even as Fleck followed suit with their newer valves, sometimes a Fleck 5800 maybe found on-line.
Although production of Fleck 5600 continues and is offered on-line, it is a several decade old design which has limitations. Even though compatible with a 2 ft3 softener, the 5600 is best suited for applications with a 3/4" or smaller main supply line diameter.
While a new piston may not be currently needed, perhaps a generic water treatment dealer that is local to your area, will continue to have some stock of regular downflow 7000 softener pistons. That dealer if asked, may be agreeable to selling a Clack WS1 or Fleck 5800 directly for self-install.
Performing a brief on-line search, I did find listed, High Flow 7000 pistons for rare, 35 gpm larger commercial applications, available from some water treatment equipment distributors and on E__y. The only 7000 regular downflow softener piston I found on-line, was listed on A____n.
If deciding to repair your existing 7000, if possible, you might purchase at least one spare of each part, so ensure those components will continue to be available to you in the future.
