mobias... Not only are there the large parts houses such as Numrich (GPC), Jack First etc., but there are also many, many small businesses that carry handgun parts. Some of these outfits cater to those who attend gun shows, perhaps specializing in hard to find items, others are a resource for local gunsmiths, and the public at large.
Like Sam, I tuned up an old *&* recently, a .44 hand ejector, first version. Parts were no problem, everything needed was obtained within half an hours drive.
Once you get to know the proprietors of these businesses you may be invited to "look in the drawers over there". There is an advantage to working with the locals: you can pick and choose among parts to select the one with the dimensions that will work best for your application. One such establishment close to me has a drawer with hundreds of parts for really old *&*'s: models #1, 1 1/2 etc. When I spotted these, and reacted with pleasant surprise, the owner said that he had forgotten he had them!
Where parts are unobtainable, or simply not worth the time required to track them down, replacements may be made by anyone with access to simple machine tools, or even, in many instances, with patience, by hand.
Bob