Short of a rstripped eceiver, any of those parts could be bought direct - thus cutting out the builder - if they told you who they got them from. That is probably a good part of why they don't say where they get them.
And that is what I'll do next time after a bit more research and study.
IF I can get any straight answers on parts source.
Part of this is an AR specific issue as well I believe. Curious.