You probably have already tried this, and I have never tried for parts for brass locos, but for old parts in general, try calling around to some of those well known, long established hobby shops. They have all manner of stuff in drawers behind the counter, boxes in the back, that sort of thing. I would think that a store that specializes, or once did, in brass would be your best bet. I've had good luck coming up with odd bits like parts for Pecos River cars, retrofit frames, and out of print decals.
I live in a place with no LHS, so I try to fit several hours of shopping into my travel plans. And it doesn't hurt to chat with one of the employees who has been around for a while, there is often a lot of stuff that isn't on the inventory anymore, hiding away somewhere, and you may get the 1979 price to boot.