OK, brief answer: you can get the standard sear block from Brownells. This will include the magazine disconnector but NOT the manual safety.

Sorry to break it to you, but the manual safety cannot be retrofitted. S&W will not sell you any parts related to safety devices. They will provide them to armorers who have graduated from one of their schools, but I'm not real sure if the parts can be retrofitted-SFAIK, the sear block is different and you need the whole item plus the safety parts.

FWIW, I lugged magazine disconnector equipped service pistols for over 20 years and find the fear and loathing associated with them to be ridiculous.