KKM makes some 357 Sig barrels for the M&P Shield. Don't know if this is what you're looking for, but thought I'd throw it out there.
KKM 357 Sig Barrels for M&P KKM is just a 15 minute drive from me. I've bought several of their barrels and have toured their facility and I'm a big fan of the company and their products. They make an absolute first rate barrel and their customer service is just outstanding.
I have installed KKM 1911, Commander, CZ-75, and Beretta 92/M9.