I totally agree with you! Marui, there's tons, that's a given. Next would be KJW? Maybe? I don't even really see KSC parts out there. It is really rare. However, T_A_N_K is right. Not really popular. I have heard that Tirador found someone a threaded barrel by KSC. Someone made note of that when I was finding a part.
http://www.airsoftcanada.com/showthread.php?t=69226
Jugglez, Does the guarder recoil spring work with the KWA P226 too?