I got the rubber all weather matts for my 500, at Christmas. Love them just vacuum and wipe off with damp rag. No rear floor mats yet for the 500. Talk to a Fiat rep, at the auto show. He had no idea, when the rears would be available. Go to Walmart, rear carpet, or rubber floor mats. Little over 5 bucks each. They will do, till the Fiat are available.
http://www.walmart.com/ip/Remington-Industries-1pc-All-Weather-Rear-Mat-Tan/15524390
They sell these in black too. Cheap
500madness, also sells rear floor mats for the 500.