To answer your question, you need an entire assembly. The old-style piano wire springs will not work on stock S&W M&P guides.
The stock S&W M&P flat-wound recoil assemblies (made by ISMI) are far superior to the old-style piano wire recoil springs in performance and durability. Different weight ISMI springs are available. Whatever you use, the spring weight should match the power of the cartridge used to avoid damaging the gun.