Check Green Mountain's website. They still might have some round barrel blanks left in that caliber that are the diameter you seek. What you are looking for is a black powder cartridge barrel that is made from good steel to withstand large loads and heavy bullets. Muzzleloading rifles are most often made from them.
A couple of the barrel makers on the board would probably make you one from 4130 if it was a straight barrel without a fancy profile. But be prepared to wait for it. I know that John Getz, LongHammock, and Rice have all made them in the past.