I expect the plug is the same one used to close up the keyhole for the integral lock. Those plugs are/were used on LE order handguns and guns ordered without the lock and are/were available for sale as replacement parts. Even if it's a different plug, it's still sold as a replacement part for those firearms sold without the manual safety.
As stated, S&W isn't going to sell any safety related parts outside the factory/certified armorer system to shield themselves from liability. It might also be linked to their ability to sell firearms in certain markets. In either case, you can hardly blame them. They're not going to flush the business over a few folks who decided they want to monkey with their firearms. If your piece is broken, use the lifetime warranty.
I believe the OP has two basic choices: sell or trade for a gun with a factory manual safety or learn to live without one. OP, you might want to fondle an example or two with the safety to see if it meets your expectations before making up your mind. Or butchering your current gun.