Since Nikon keeps adding to their self-service manuals, I've added pages for cameras/accessories and lenses that tell which products have manuals. Some parts are available from NikonUSA (currently 1683), but note that for truly serious internal repairs, you sometimes need access to "core" components, which aren't always available here in the US.
This is truly "void your warranty" territory and not for the faint hearted. Moreover, if you don't have a set of JIS screwdrivers or don't know what that means, you probably shouldn't even contemplate repairs on your own.
After completely scorching NikonUSA years ago when they basically closed off all third-party repairs and stopped selling parts, I have to commend them for visibly re-entering the fray (though some of that is motivated by right-to-repair laws that have come into effect).
Other site note: now that we have over 100 mount adapters, I've highlighted the ones that are autofocus on my Z-mount adapter page.