Last time I replaced mine I got the replacement Spring from S&W. Brownell's or Midway might sell S&W Factory Springs as well.
You can also get replacement Springs from Wolff Springs, HOWEVER, despite what their own instructions may or may not say on the package, MAKE SURE THE OPENED END OF THE SPRING FACES REARWARD!!!!!!! They had the incorrect instruction printed on the packaging for a long time and were causing guns to jam up tighter than a clam! This actually happened to two of my friends and it screwed up their M41's!
If the spring is installed with the open end forward, it can and most likely will work it's way through the plunger hole and jam up the gun.
AGAIN, open end rearward (opposite a1911's spring)!
All that said, they might have corrected the instructions by now - assuming you get a newly packaged one. I spoke to the owner about this and suggested he finish BOTH ends of the spiring with closed loops (then orientation would not matter), but he said that was not something they were willing to do. Wether that has changed or not i am unaware.
NOTE: S&W 7.5 pound Factory Springs are finished on both sides - orientation does not matter.