Tennessee Valley Muzzleloaders, and his father-in-law across the road that sold him the business, is an excellent choice for parts, kits, and finished guns. I will personally vouch for the quality of their stuff.
Are you going for a "poor boy fowler" or something a little finer grade? Heck, even the cheapest guns usually have Siler lock kits in them, built and tuned in-house.
Across the road is Tennessee Valley Manufacturing, currently owned by his pa-in-law. I've bought a few kits from them. I have bundled orders with several friends totalling about 11 guns and no one, not a single person, has had any complaint.
Just stay .50 cal Barrett distance from Early Rustic Arms. They may be cheaper, but they are pure crap and the owner will call you every filthy name in the book when you ask them for help with correcting their screw-ups. Shoot me a P.M. and I'll tell you what I really think of 'em.
