The City of Folsom has a strict no field use policy from November 1st to March 1st.  If you are looking for fields there are a few options:

Cordova Fields
They have lighted and unlighted fields available for rent. The Mather Sports Complex has turf fields. 
     - Softball Fields:  here     
     - Ball Field rental info: here                
     - Softball Specific rental info: here 

Orangevale Parks & Rec 
They rent the softball fields off of Hazel
     - Web Site: here                
     - Generic info:  here (pdf)
     - Phone number:  (916) 988-6140
     - E-mail: orpd@sbcglobal.net 

Arena Softball in Roseville. 
Indoor facilities available for rent.  It also has batting and pitching tunnels.  Looks very cool for parties too.
     - Web Site: here               
     - Rental Info:  here               
     - Photos of the field: here