Denton Castings in Brandon, Iowa did a might fine job on a one-of-a-kind brass casting for my RR. I designed it and gave them a B/W drawing (as well as a copy of a bitmap file on floppy) which they then sent to a place that etched the "positive" for the mold. That part was a bit expensive (around $90 if I recall correctly). They then made the sand mold and cast the piece for me (about 3.5x10.5 inches). They did it for the cost of the materials at about $50.00, (kind of a "let's do something nice for someone." type of thing) but that was a long time ago.
Are you familiar with Robert Dustin (860) 923-0174, mailto:[email protected] ? He has a catalog for $4 and specializes in etched locomotive plates. He is a regular advertiser in Live Steam & Outdoor Railroading and has an excellent reputation.