You can find those pins in a scrap yard. I know for certain they are fitted to cinqs and punto and obviously mareas and bravas. Most scrap yards would charge a pound or give it you for free. Some have a £5.00 minimum though.
Or some one on eBay has 5 for sale
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=370307148398
Your fiat dealer can provide any part, they might be in a bag of 5 or 10 though so you would have to buy the bag.
The part number for that pin is:46737923
Make sure the C shaped bracket that the pin goes through on your car body has not snapped. Mine snapped in the middle, that allowed the bracket to bend the pin and snap it. I got a replacement from both parts from the scrap yard.