You didn't say what calibers the rifles you have are but, if you want to get a pistol that shoots the same ball size, get a single shot pistol in the same caliber if they make one.
These use a patched ball although most people use a thinner patch with the pistols.
The thinner patch works because the powder charges are a lot less and they make loading the pistol a lot easier than a thick patch like a rifle uses.
If you don't mind buying some different sized balls for your pistol any of the cap & ball revolvers will do. Try to get a steel framed revolver rather than a brass framed gun.
The steel frame can take a lot heavier powder loads without damaging the gun.
The difference in the ball size with the revolvers is because the ball must be larger than the bore size and is rammed bare into the chambers in the cylinder.
Where a .45 caliber rifle might use a .440 diameter ball with a patch, the .44 caliber cap & ball pistol will use a .451 to .457 diameter ball.
Likewise, a .36 caliber rifle might use a .350 diameter patched roundball but a .36 caliber revolver would use a .375 diameter ball.