You are pretty much in the position where you have to make it, find it used (gun show, used parts dealers, worth collecting a few business cards), or find a modern replica that might have parts to adapt, like the Italian made replicas of the 1885 (dunno if they make a flatspring version or not). If the modern replicas parts might be usable, then start with Gravel Agencies and Western Gun Parts. Have make model, and part number in hand when calling either (for the replica).
Used gun parts outfits tend to be a guy or guys that have been hoarding goodies for a long time, and are worth finding.
Cheers
Trev