On eBay, 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 D850 parts" brings one set of matches, while typing "nikon D850 part" (singular) brings up a different list. 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.
