A lyman great plains rifle is sold in flint and .54 for $350 to $425.00 other than that there isn't much currently made as a production gun. About 25 yrs ago, the CVA Mountaiin Rifle was available in 58 cal. (called the BIG BORE) Although they made them in flint only one or two years.
Jim Chambers make some really nice stuff. There are other semi-custom and custom "stockers" out there. You pick the components and they will inlet and finish to the degree you want. I have seen some of Brad Emich's stuff and had him fix a custom rifle of mine. His shop can be found on line, Cabin Creek Muzzleloaders. Don Yerks makes some nice stuff, Bill Slusser, etc.
There's dozens more, that just what I can think of right now.
As stated above, if you want a quality hunting rifle, you don't want all the "flash and scratches" that come with fancy shiney inlays and engraving. My custom rifle is extremely plain, not even a butt plate, but it has what counts, good double sets, a fine tuned flint lock and a straight tapered barrel, with browned iron thimbles. It is very well balanced and lighter than many production guns.
Such rifles were common in frontier PA, and were referred to as Schimmels, "barn guns", very plain and functional and few examples survive because of the rough service they saw.
Chuck Dixon of Dixon's Muzzleloader shop in PA, is a bit of an expert on Schimmels.
For just the quality components of a semi custom gun, keep in mind that a lock will be $100, a tapered or swamped barrel will be $200 to $300, quality double sets will be $60.00 and a stock will be at least $100 for plain wood.
You are already up to $460 to $560.00 before any labor is involved. Better components will run up the price even more. Having the builder "tune" the lock adds immeasurably to the function of the lock and reduces ignition speed to the point that the gun will go off as fast as a cartridge gun. Tuning a lock is well worth the time and cuts down on the frustration of shooting a flinter. A good flinter never goes click, hisssss, bang. It just goes bang. If you ever experience such a gun, you'll wonder how other folks can put up with a production gun.