Magwell - Speed Shooter Specialties
Mag Pads - Arredondo and Taylor Freelance. Arredondo are easier to service. Taylors are a little nicer, machined from aluminum.
LH Holster - I use a Hogue PowerSpeed, modified to work better with the M&P.
Front Sight - Dawson Precision with a red FO rod
Rear Sight - Warren Tactical
Guide Rod - Stock
Grips - Courtesy of a soldering iron
A lot of my parts came from right here, in the classifieds, otherwise, Speed Shooter Specialties is a good source for S&W pieces.