You might want to check with Numrich Arms for a barrel. They did have some originals once, and also some barrels for the H&R Trapdoor repros, that might also work.
Another choice would be to reline the barrel, and have the pits polished out, if they're not too deep.
If you end up getting a new barrel blank fitted, have your smith check on a Green Mountain BPCR barrel. I had one fitted to my old Ballard #4 Perfection in .45-70, and it is an excellent barrel! I ordered a second one for my restoration of my Remington Hepburn Creedmore rifle, as I was so happy with the first one! They're one of the most reasonably priced barrels made. brownells carries them, or you can get them directly from Green Mountain.
Hoe this helps.