STANCE Nissan Z31 GR+ Coilover - Touge Factory Store
Ground Control will also build you a set of coilovers for your Z, similar to the link you provided. They mostly prefer koni and eibach springs. Basic problem are the dampers cannot hold higher spring rates.
There is also the option to order from a few companies that fab them out of j-land as well.
Your best bet is to just buy stance. They also offer a weld-on service if you are unable to fabricate.