Anyone know where I can buy one? Brownells has let me down.
Here's several sources. The same spring is used in all PP series guns, including the PP, PPK, and PPK/s:
http://www.jackfirstgun.com/
You have to call.
http://www.poppertsgunparts.com/cgi-bin/webdata/webdata_gunparts.cgi
http://www.carlwalther.com/
Preferred source.... all brand new genuine Walther parts.