S&W was the barrel maker. When they quit making the magnum K frames, they quit making barrels. If the factory doesn't have any more, you're stuck with shopping places like Numrich/Gun Parts Inc.
You can always get your 6 inch barrel shortened to 4 inches. The front sight would have to be relocated and the make and model/caliber markings on the barrel will no longer be centered, but it will work.
Alternatives:
If there is sufficient demand, the factory might consider making a run of spare barrels. Don't hold your breath.
Should a distributor special order a run of magnum K frames, there will be a small number of extra barrels available. Again, don't hold your breath.
If money is no object, you can always take the piece to a custom gunsmith and get a quote on carving a new barrel from a blank. Take a bank reference.