Most of the small pocket revolvers were called .31 caliber after the size of the bore in them. Describing a muzzleloading gun or cap and ball pistol by the bore size was common back in the day.
Your pistol should shoot a .321 diameter lead ball. A ball this size or slightly larger will totally seal off the .319 diameter chambers in the cylinder and come close to matching the groove size in the barrel.
You can try loading a larger ball like Justin44 suggests but you may find that it is just about impossible with the short loading lever on your gun. Actually, loading a .321 diameter ball in one of those .31 caliber pistols can be a chore to do using that little loading lever. That's why I suggest, if you do use a ball that is larger than .321, remove the cylinder from the gun and load it using the bench or a block of wood to support it. A plastic mallet can be used to tap the balls into the chambers.
Your powder charge should be just enough to fill the chamber leaving just enough room for the ball at the top.
3Fg or 4F black powder can be used in these small pocket pistols. If you can't get real black powder, Pyrodex P will also work.