Wikipedia indicates that there are different models, calibers and patent dates that were made in different factories.--->>>
https://en.wikipedia.org/wiki/Ethan_Allen_(armsmaker)
Even the single shots had different top hammer and side hammer designs.
This Allen & Thurber nipple from Numrich doesn't specify if it fits a repro. or original model. --->>>
https://www.gunpartscorp.com/products/522540AE
I don't know anything about the parts that you listed and how important they are, or if they need to be originals.
One can only speculate about how to either search for parts or an antique gunsmith who can fabricate parts to put the gun in working condition.
The main idea would be to contact an antique gunsmithing outfit, antique gun dealer or antique gun collector who may have leads about either fixing the gun or trying to track down the parts which is a long shot.
An example of an antique gunsmith is Lodgewood Manufacturing. They don't list Allen & Thurber parts but may be able to provide help to fix it. --->>>
https://www.lodgewood.com
Another example is to contact an antique gun dealer and ask if they know of any antique parts dealers or gunsmiths who may specialize in pepperboxes, or who can fix a pepperbox. --->>>
http://www.davidcondon.com/index.htm
A third suggestion would be to contact your state, county or large city Antique Gun Collectors Association, or Gun Collectors Association and inquire about antique gun dealers or antique gun parts dealers who may specialize in antique revolvers such as yours.
These collector/dealers run their businesses out of their homes, and attend antique gun shows, they buy estate guns and collections, and deal with other collectors in their association. Some may accumulate parts or parts guns. Or they may know of gun parts dealers to ask about parts.
Think of it as hunting for a rare antique car part. There may be an undiscovered part somewhere, or someone may have a parts gun to part out.
Not everyone lists all of their inventory online like salvage yards do.
And I don't know of any clearinghouse for antique gun parts.
Some folks would keep an eye on eBay for possible antique gun parts listings using multiple search terms.
Could you tell us anything about how you came across the guns?
Can you show us any photos of the guns?
Welcome to the THR black powder forum Paul69usmc, and good luck with your parts search.