Depends on the part, depends on the model, depends on the company. Some companies won't even sell parts to gunsmiths except their own authorized repair services. Some won't sell certain parts which they feel have to be installed at the factory for one reason or another. Some won't sell some parts because they don't have them for obsolete models.
Some will sell parts to parts dealers (Brownells, Gun Parts) but don't want the nuisance of dealing directly with the public for reasons of cost.
If you need a specific part, I would say to go to web sites like Gun Parts, Jack First, etc. If they don't have the part and it is for a current gun or for a reasonably new gun and the company is still in business, call, write or e-mail them with the model, serial number and the correct nomenclature for the part. If the nomenclature is unknown, provide a picture or drawing of the part.
If the company is out of business, and parts are not available on the parts market, the part may have to be made. Sometimes, a simple substitute will do; every gunsmith has made pins from a piece of drill rod or even a nail. But for a complex part, the cost would be far too high for the average customer, especially if it would exceed any reasonable value of the gun. (Example: Would anyone pay $300 to have a gunsmith make a hammer for an RG-10?)
Jim