I lost that piece of a stock on my old rifle, I never managed to find a replacement.
Your best bet is to just buy a whole new stock, ought to be about £20, I'm not sure anyone knows where to buy the spring and pin set that hold the lever on.
Just search for an M4 LE Stock for whatever brand of M4/16 you want it for, I imagine you're after a KWA one?
That's as much as I can help unfortunately.
Though you could try and make one from scratch, you just need a screw, a nut and a spring. It's getting them all the right size that's the hard part.
Welcome to the forums by the way.