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.