SP Historical Society and/or SP Modelers.
http://espee.railfan.net/espee.html
http://pages.tca.net/lruback/sphome.htm
http://www.sphts.org/
A good book for yard maps is "North American Railyards" by Michael Rhodes (St. Paul: MBI Publishing, 2003). It has maps and/or info for the following SP yards (listed in the UP section, though):
Taylor (no map)
West Colton
Eugene (no map)
Roseville (Davis)
Englewood (no map)
Pine Bluff (SSW - no map)
Mike Lehman
Urbana, IL
"No soup for you!" - Yev Kassem (from Seinfeld)