Most gun makers will not sell critical parts because of safety and liability reasons, even to actual gunsmiths.
If as example, some Billy Bob bought a hammer for a gun and botched installation it would be dangerous. Of course, the maker would be blamed and sued if there was an "accident".
So, most key gun parts are listed as "Restricted" which means they're factory installation ONLY.
Why not sell to "real" gunsmiths?
WHAT is a "real" gunsmith?
All you have to have to be a "gunsmith" is a Federal Firearms License to be in business.
There is no standard anyone has to meet as to actual qualifications or training.
You can get a license as a gunsmith and appoint your dog as the gunsmith.
Since a gun company has no way of determining qualifications, most critical parts are sold only for factory replacement.