One source for knobs I saw listed in an article was Reid Supply Company. (I have no experience with them.) You might also want to try Richard Ritter for brass knobs. You can Google either for their respective websites.
As alternative, you could also try the For Sale section of this forum or of the LF Info forum as well as FleaBay for listings of parts cameras, aka Hanger Queens. There are a few auctions going on now in FleaBay - none of which I have any vested interest in - for beater or parts Ansco cameras. As Ansco cameras can have either Bakelite knobs (in red or black) or metallic (brass) knobs, be sure to examine the listing pictures when going after these 'donor' cameras.