1960's model or recent 500?
For vintage parts its best to join a Fiat group on Facebook and ask.
I presume it is for the latter 500, and your best bet is to call scrap-yards in your area. They will know who buys in Italian cars and would be able to assist further to hunt down a mechanism.
What went wrong if I may ask?
Reason being, if I see a Abarth 500 cc secondhand for a good price I probably end up buying it for the wife. Nice weekend toy and wife wanted a 500 for so long now - no more prams to lug around.