To the best of my knowledge, we have no equivalent to an Italian "Raduno" in this country. In Italy they have "Cinquecento specific" Radunos (I am going to one at Misano in September)--normally just a 'one-day' event in a town square.
Some of these one-day events have up to 200 500s, of all varieties, attending. There are so many in Italy that you would probably be able to, by contacting the "Fiat 500 club of Italy", tie in a holiday in Italy with a visit to a Raduno.
However, I have been led to believe that the days when there were stall after stall selling Fiat/Abarth/Giannini parts are gone. It would seem that Fiat has 'got the hump' and is clamping down (HARD) on suppliers of 'pattern' parts--not only for Fiat but also for Alfa-Romeo and Lancia