If you don't care about dovetailing the barrel, you can do that yourself -- I used to make muzzle loaders, and dovetailed many a barrel.
In fact, you can also make the sight yourself. I had some old angle iron that I made dozens of sights out of. I simply made the base by filing a rectangular piece to the right size and angle. Then I drilled a hole in the base (the male dovetail), filed it rectangular, and fitted the front sight blade with a little "tit" that matched the hole. A little silver solder, and presto! a nice-looing sight.
To cut the female dovetail, mark it with Dykem or a similar marking substance, then rough cut a slot wth a hacksaw, finish with a file. Take a 3-corner file, grind one face smooth, and use that to cut the overhangs of the dovetail.
File from both sides of the barrel -- this will give you a slight constriction at the center of the dovetail and hold the sight tightly.