The Army had not been "fighting on the plains" long with the .45 Colt cartridge. They issued .45 Colt ammo for only a little over a year before adopting the Schofield and from that time they issued only the so-called "Schofield" cartridge. The Army got all its ammunition from Frankford Arsenal and they made no .45 Colt ammunition at all after 1874.
There were very few Schofields sold on the commercial market; almost the entire production went to the Army. Sales of Schofields as surplus began in the early 1880's, but only a few had been available on the commercial market prior to that and ammunition would likely have been hard to get for at least a few years. I think that if I had a Schofield in the 1880's and was going to any remote area, I would have traded the gun in on something for which ammunition would have been more available. (Think trying to find .45 GAP ammo today!)
Jim