You can get SIG parts from Brownells.
I suspect you’re thinking of the modified mainspring they came out with within the past few years. But maybe not – I believe it’s pretty much standard issue in their newer pistols. Anyway, it’s about half the length of the old spring and sits on a long plastic housing.
AFAIK, it will not work with the older grips without some Dremeling (the plastic housing gets in the way).
If you check the Brownells website you can find some exploded diagrams of various SIG pistols which should show you what I’m talking about.
SIG also has changed the trigger bar return spring, but if anything it’s heavier. It also won’t work with the older grips.
Alternatively, they may have a spring I plain and simply don’t know about. You might want to just call them.