They are out there;
A lot of places are out of stock after covid, this place has one left but it is percussion.
https:// www. grafs.com/retail/catalog/product/productId/ 10266
Kits, there are kits and there are kits, the assembly kits like Traditions are easy, the parts kits take some serious skills to complete, these would be Chambers, Pecatonica and Track of the Wolf kits.
The difference is with the parts kits you get a historically correct copy of a rifle made between 1760 and 1810 depending on the style. Chambers makes the best parts kits but none of the parts will fit in the precarved stock except the barrel, the pre-shaping is done but undersized so you have to inlet every part into the wood for a correct fit.
Precarve kits like Pecatonica can be real iffy, I got one and spent more time fixing flaws that their duplicator made than it would have taken to make a complete rifle from a plank.
Lyman or Pedersoli are close to a plains rifle but not exact, Traditions are way off the mark. The locks in the parts kits are far superior to the assembly kits which are OK but may have issues.
https://www .flintlocks.com/
This place has some;
https:// abes-general-store-co.myshopify.com/collections/muzzleloaders
If you just want a gun to shoot and don't care about historical accuracy or have the gunsmithing skills to put a complex kit together go with a Traditions, Pedersoli or Lyman.
They make two types of barrel rifling, one for round balls and a patch and one for conical bullets. Slow twist, 1 twist in 48" to 1 twist in 60 and up are for round balls. Fast twist, I twist in 28 to 32" is for conical bullets only.
A Lyman Great Plains rifle is a slow twist rifle made for shooting a patched round ball
A Lyman Great Plains Hunter is a fast twist rifle made for shooting a large conical bullet.
All the factory rifles have a model for each projectile choice, most have a right hand left hand choice as well.