Just an FYI re CA and the full size M&P - they're no longer "CA Compliant" meaning they can't be brought into or sold in CA via the usual FFL route. They can be bought used and in "non-CA" configuration if already in state. Usually for a fair amount of $$$. Like around $700.00 - which is what I paid for mine a few months back.
FWIW, I never had any difficulty finding 10 round magazines. Haven't bought any, but see them with Greg Cote, CDNN, Midway with no shortage.