You may hafta' give up on that extra cylinder thing. BP cylinders are, and must be, built to the specific specs of the weapon. Since this varies widely between manufacturers, you can't expect a Pietta or Uberti or even an original to fit your revolver. You would need to find one from the same batch as used in 1976, unless you know a really got gunsmith who can custom make one. This is also a safety issue. Perfect fit is manditory for safe operation.
New 1858's are affordable, the extra cylinders relitively cheap, and are sold in about every gun shop. This will cost you less than having a custom cylinder done up.
I have a 1973 vintage Navy Arms Remmie. I can't find anything for it any more, so it hangs on a peg above the fire place. Every spring in it is repaired, some twice, the hammer hole sleeved and rebored, the trigger sear long ago replaced and rebuilt. I didn't abuse the weapon, I just fired it about every weekend for fifteen years. Were it a new Pietta I could buy the parts right at Cabela's!