If you can't find the official gear, you could roll your own by starting with 2 longer rods and a longer jackscrew.
https://www.mcmaster.com/#98935A703 is a 12 inch jackscrew, and
https://www.mcmaster.com/#6112K44 is a 200mm (8 inch) rod in the original 8mm diameter.
That said, I'd think rolling your own X upgrade would be easier than rolling your own Z upgrade. Longer bars, bigger bed, and some belt should do it.