Here's the spring:

S&W M&P Mag Safety Lever Spring [1981] - $2.00 : Welcome to Speed Shooter Specialties

Here's the lever.

M&P Sear Disconnect [1786] - $2.00 : Welcome to Speed Shooter Specialties

Or, you can post a Wanted to buy (WTB) in the classified as many take them out and maybe will sell or even give you the parts. Plenty of vids about removal on YT. Install is the reverse.