Dont know squat about Brown Besses, or where to get parts. But building a gun I can help with.
The word “kit” can be very confusing and misleading.
(boy is this part gonna ruffle some feathers)
Muzzle stuffers are basically in 3 categories.
Inlines, the curse of the muzzle loading world. They are around so hunters can kill more deer.
There is nothing that relates them to real black powder rifles. They art not part of this thread.
Third world production guns: Kits that are made from cheap parts, not historically true.
If you buy one, you will replace parts, and you will not be able to buy them from the maker.
You will have to get them from Dixie and make them fit. Over the last 2 decades they have gotten better. I have worked on others, and a lot of them.
This does not mean they were made in a third world country, those most are either 2rd or 3rd.
But the manufacturing is crap.
High quality kits:
Jim chambers: He is a great guy, and their shop is top notch. Even if you screw something up. they fix it. One of the screws in my lock plate had too much browning on it and rusted into the hole.
Mr Chambers not only fixed it for free, but told me never to sell that Chamber/Rice lock.
So my grand son gets it…someday
Muzzle loader Supply: Where I bought my parts, so I cam a tad biased.
My Sheetz rifle is retired, but you can still get the parts. How they differ from Chambers kits is that you get to choose what grade of parts and color and style. My rifle has parts from a much older flinter on it. Why? Because they looked cool. A purest told me to tell folks they were from the family gun that broke and was transferred to this one, an heirloom of sorts. I just tell folks they are cool looking.
Check out the, but call the girls, and see what options are available that are not on the website.
I stay away from Track for rifle parts, but just about everything else I get there.
Flint VS Cap:
Cap guns are for kids. Do they work sure, are they better, heck no. Cap guns have their own problems.
But then you are asking about firearms that were made 50-80 years before cups came along in 1836.
My Browning said: “They never made a rifle better, only faster” Think about it. Is the M16 better than the M14, or M1, or A303? nope only faster and lighter.
Flintlocks: The learning curve is vertical. Throw everything you know about ignition, and become a school boy/girl again.
Some say you cant shoot in rain and wind. Any one ever been to the Cascades mt range during hunting season in the fall? Rain and wind and snow are always on the table.
When you are seasoned with a flinter, NO weather will stop you.
A crayon seals the frizzen to the pan. a condom over the muzzle keeps the FOD out of it.
(learned that in Nam). Since 2005 I have never had a failure to fire. (cleaning and lubes are a whole different thread).
I hope this saves you money. We learned the expensive way.