I did not know that the mags were s.s... Are others that are just blue?
The mags appear to have the same Melonite coating as the standard M&P slide. I've never seen M&P mags that appeared to be blued or natural SS.
To more directly address the OP's question...
The S&W website shows a Talo distributor-exclusive M&P9 FS and M&P40 FS with nickel-boron slide finish.
You will have to contact Talo, look on their website, and/or call your local gun stores to find out where Talo guns are available.
As Ploufedaddy mentioned, IIRC the M&P9 FS and M&P40 FS were available as a Cabela's distributor exclusive in 2009 with stainless slides. AFAIK these guns simply had a natural satin SS finish rather than a nickel boron coating like the current ones; however, I had trouble finding out the straight dope at the time.
Ploufe, I've not seen a nickel boron M&P45; is that custom?
Although this may be slight thread drift, the FDE frame with black slide has been offered on various flavors of the M&P45 for several years now, but it's only been offered on the M&P9 FS and M&P40 FS as part of the Cabela's distributor exclusive Range & Carry Kit; FWIW many of these kits were shipped with black backstrap inserts due to a shortage of matching FDE inserts. AFAIK the FDE frame and
slide has only been offered on the M&P9 VTAC. Finally, AFAIK no two-tone version of the M&P9c, M&P40c, Shield, or M&P357 of any type has been offered.