NikonUSA seems to have backtracked a bit and has starting offering a few parts to its customers again. For the Z's, you can look at this NikonUSA mirrorless camera parts page. Unfortunately, at the moment, only the Z6/Z7 battery door replacement is available, but let's hope that Nikon sees fit to add additional parts in the future. Other useful parts would be replacement eyecups, replacement side panel doors, and maybe replacement grip covers.
On eBay and Aliexpress, you can often find almost all parts for cameras. If not immediately after launch, typically within a year or two after launch. But you'll need to be somewhat careful about how you type your search request, as eBay's current algorithms are not great. For instance, typing "nikon z6 parts" brings up only one match, while typing "nikon z6 part" (singular) brings up over a dozen. On Amazon and eBay, using the term "replacement part" usually helps refine searches, too.
I can't really recommend that you try fixing major problems, particularly ones that involve internal disassembly, but grip rubber and maybe even rear LCD and top panel replacements are probably within the realm of a well organized, careful, skillful worker.
The rubber gaskets on the Z6 and Z7 models are named 126ZB (for the USB/HDMI/remote cover) and 126ZC (for the headphone and microphone cover). These parts are easily ordered, though they're a little fidgety to replace. Browsing Amazon, eBay, and Aliexpress I was also able to find battery door replacements, top covers, rear covers, buttons, power switches, hot shoes, SD card doors, LCD displays, shutters, and even the main digital board and image sensor.
