OP: I bought 1 - 3" lowering keys and a pair of lowering brackets for the rear for just over $150 from airbagit.com.  The keys for a C3500 are similar but are clocked different.  Bracket was $109, keys were $49, free shipping.
      
                        
                      
                             
      
            
            
        I got the brackets and keys from airbagit.  Brackets fit fine, look to be well made.  Will report on stance, etc. after I get the keys installed.