I bought my Pop in 2008. When I collected it there was no exhaust cover. If you go onto the Fiat website and look up accessories you will see a 500 Exhaust Tailpipe. It is quite original (I certainly haven't seen many about) and I love it. A bit pricey at £99 but I was lucky enough to get Fiat to pay for it and have it installed when I bought the car, due to a mix-up. Hope this helps.
If you check the U.S. based boards, there are plenty of people who have done it. I had thought about it, but figured it's not worth the money (let's face it, you're not going to get too much out of these engines with a few bolt ons) and I've listened to them on Youtube (from people who have installed them) - I don't want my car to sound like a buzzy lawnmower
I've fitted a Remus sports back box and Remus race tube (an unsilenced centre section) it was TOO LOUD! I've since had a Powerflow silencer grafted into the race tube which has made it sound just right. Remus used to do a silenced centre section but for some reason discontinued it. Everything is stainless and came with a lifetime guarantee.
I posted a video on You Tube of the sound before I had the centre silencer installed. http://youtu.be/oS4suZrYBAY