Just out of curiosity, I checked the S&W site -- since the SW99 and the P99 are "cousins". S&W has a detailed parts list, but no exploded parts diagram to show what the part numbers correlate to! It does list a "trigger return spring" for $2.35, but I have no idea where it fits in the gun...and there isn't a corresponding part with the same name on the Walther diagram.
If you can find the SW99 exploded view, you might have more luck!
From S&W's web site, I sent an email asking for a parts list. My email was answered very promptly:
We can send a parts list if you let us know your address.
Our parts dept can be reached at 800-372-6454
I sent them my address...
If your in a hurry, you could call them at the above phone number...