You can still get the bits from Fiat, but the prices will amuse you. For a seicento, the abarth skirts and spoiler come to over £300 from Fiat, not exactly a bargain.
I picked up two skirts and a spoiler second hand, for a fraction of that. You can either try looking for a second hand set (I suggest ebay, loot.com, forums, exchange and mart, car breakers etc) but failing that you'll have to buy them new.
If you do buy them new, try looking in the back of a porn mag (sorry car mag) that are Max Power, Fast Car and Revs. After you've flipped through all the naked women and adverts for 0891 sex lines, you should be able to find a few adverts for car bodykit shops selling cinq bits. These will have your parts, and will be cheaper than Fiat, although they probably wont be OE Abarth parts.
Hope this helps.