Since the U.S. military didn't use the M1878 Colt's DA revolver, I don't know of any "issue" holsters that would work with it, although the later New Service ones might fit. You'd probably have to have one custom made. The problem there is that a pattern would have to be made, and it might mean shipping the gun to the maker, unless he happened to have an original. A possible alternative would be to see if any holster that fits an N-frame Smith & Wesson would work, and go from there. If the N-frame holster works, it would simplify things greatly. The presence of the ejector housing would probably mean wet-fitting the leather, but that's no big deal.
I personally have never had a request for a holster to fit a M1878, so I don't have a pattern for one.
Good luck!