You can buy top quality kits from Track of The Wolf, Dixie Gunworks, Tennessee Valley Muzzleloading, Jim Chambers, Jim Kibler, and others. Of course, you can buy a finished rifle from any of these places, too. They have websites. Look them up. These are top quality and not cheap.
Pecatonica Longrifle Supply doesn't sell "kits", but they do sell all the necessary components.
I have built four longrifles myself; one flintlock and three caplocks. I'm considering getting the parts from Pecatonica to build another flintlock, but right now I just don't have the time.
Then, there are plenty of custom builders, such as Hershel House, Mike Miller, Clay Smith, and Lowell Hearer who will make a beautiful rifle for you. But, you can expect to pay $2000. - $5000. for one of their historically accurate firearms.
Then, there are the artisans who specialize in muzzleloading accoutrements. Shot bags (sometimes called a "possibles bag", although I personally dislike that term)(I made my own bag), powder horns (I have a beautiful "southern banded powder horn" made by custom horn maker Ron Hess of Georgia. However, I also have several powder horns that I made myself), custom knives, colonial clothing, etcetera.
There is a whole world of traditional muzzleloading out there. Go and find it.
For over 25 years I have done all my hunting solely with traditional muzzleloaders. I mainly use a .50 percussion longrifle that I made myself for big game, or a .62 flintlock smoothbore fowler made by Caywood of Arkansas. I bought the fowler "in-the-white" and finished it myself.
I admit that I am a muzzleloading snob. I absolutely despise those abominable in-line things!