Midwayusa used to have M & P 357 Sig barrels but I couldn't find one on their site. I did find it here though;
GunBarrelSupply.com: Storm Lake S&W MP Barrel, SW-MP-357-425-DI-0P-00T, 357
I'm sure there are other internet gun supply sites that have them.
I would buy one for my MP 40 but I spend too much $ on different calibers of ammo now, like 9mm, 357 mag, 40 caliber, and 45 caliber.
I'm gun and ammo poor!
After posting this I went back to Midwayusa and found a storm lake 357, M & P barrel but it is $20.00 higher than Gun Barrel Supply Co. I did not check shipping cost though.