.357 sig is basically a necked down .40 SW supposed to be similar in power to a .357 magnum?
They use the same mags, should just be able to swap barrels between the 2 calibers, however your sights may be off because of the different ballistics between the 2 calibers. CDNN investments may be able to hook you up with mags, and the may have these pistols in stock. If not check the EE or Shotgun News for ads and have a dealer order one for you.
CDNN had LE marked demo mags for $18.99 down from the crazy $70 prices I have seen. You may be able to find a used police trade in pistol in good condition with holster wear. The only flaw I have seen with these pistols is the Blueing on the (stainless) slide wears more than others I have seen.
Good luck,