unable to find a tension spring for this platform to cut down on the recoil
The recoil spring must be matched to the particular ammunition you are using, and will NOT "cut down on recoil." A too heavy spring will not eject properly, and a too light spring will batter the frame. The recoil spring is selected for gun functional reliability.
By the way, the recoil springs for the 9mm and .40 SDs are the same part number.
IMHO, the stock S&W recoil spring assemblies for the SD are as good as you are going to get, unless you handload to low speeds and need a lighter recoil spring to get proper ejection.