Try Numrich gun parts. If anyone has one, they might. The 6" version was fairly uncommon. Nickle was not the norm. A factory nickled 6" M-28 would be right up there with hen's teeth on the rarity scale.
You would be better off geting a blued 6" barrel and having it nickled to match.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball
You can find barrels in the parts sections of guns america, guns international,auction arms,gunbroker.Like said above the nickle versions are not very common and finding one is gonna be a tough job but book marking these sights and surfing through them may bring one up.