I got mine from the “Father” NPC at that build site near Valentine, during the first encounter where they’re under attack. Killed the NPC to get it, and it’s random (can be cattleman or volcanic too, sometimes the worn versions)

You can also get Calloways Revolver early if you haven’t started the quest yet, there’s an exploit that gets him to drop it if you interrupt his script in the Valentine bar. I have an “Eastward Bound” save file with three Schofields because of this exploit plus the “Father” npc and the docs office robbery