BCM and Rainier Arms are two of the best parts suppliers. Rainier Arms also has a quality gunsmith service that can build out your parts picks for you. Between the two they cover so much of the quality parts market for AR's (if these two don't carry it, you don't really want to buy it) it makes little sense to buy elsewhere...
The recommendations above does NOT include custom barrels (Kreiger, Douglas, Lilja, LW, et al) nor optics and their mounts. You can buy very good barrels from BCM and Rainier, but go to the barrel makers for special needs. Ditto for optics and mounts, for more variety go to suppliers that specialize in these items, like SWFA, Liberty, CS Tactical, EuroOptic and others.
National Match parts and service for AR's is another specialized area, and outside the area being addressed.