I usually get Service Packs from Wolff if I'm going to take down a S&W, probably can get them for the M&P also.
Springs for SMITH & WESSON NUMBERED SERIES Semi-Auto Pistols

You shouldn't have to worry about pins for the 3rd Gen gun but the M&P has some roll pins that you might want to have on hand. General parts or pins can usually be had at Brownells or try Speed Shooters Specialties.
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Dave