Wilson Combat sells the kit. The flat wire springs use a smaller diameter guide rod. The kit contains the correct sized rod. The Wilson FW guide rod has a sleeve sized to the hole on the regular WC buffers so they can still be used. Pretty reasonable.
I recently bought a Sig 1911. It has the flat wire setup in it from the factory. Haven't got it to the range so I can't give a report. I did order up a spare 17 and 20 lb. flat wire for the Sig to keep in inventory. Supposed to be softer shooting and much longer life. My buddy put the FW kit in his S&W and loves it. Flat-Wire Recoil Springs/Kits-http://shopwilsoncombat.com/
I have a bunch of Wilson parts on my SR. They are top shelf. I'll probably put the flat wire kit in on the next spring change also.