I would check the classifieds at the
Traditional Muzzleloading Forum because All of the factory rifles out there have barrel markings from the modern world, and none are close copies of original rifles... though the Lyman GPR is the closest out there, but it's not a longrifle, it's a plains rifle of the 19th century.
I would also check the used rifles at
Track of The Wolf.
And I would check
Armslist, and use the word "flint" in the search engine. Sometimes you can find a good bargain on Armslist as the seller doesn't know what price to ask and you spot it quickly...as bargains get snapped up fast.
On Gunbroker, you can find some flinters, but in the 18th century rifles and muskets, usually the sellers are living in a world of fairies and dragons they are so overpriced. For example there are two full sized Brown Bess muskets, used, being offered, one is more than $900 and the other is over $1000... when in kit form they normally go for $900 and thus carry the warranty. One fellow wanted $1250 last month for his used Japanese made bess repro... for that much you can get an Italian made one brand new.... as I said, the world of fairies and dragons....
You are probably looking at $800-$1200 in a used longrifle that meets your criteria. Under $800 are sometimes found, but it's rare. I wouldn't pass up a good, used, Dixie Gun Works, Tennessee Rifle if it was in .45 or .50 and priced at $700 or less. The modern markings on the barrel can be polished off, and the barrel re-browned. Sure, it's also an early 19th century style, but you wouldn't get too much flack for it at a Rev War event.
LD