In the USA, they periodically come up on E-Bay.  
The 500 Nuova and 500D were imported to the USA, newer models were not,
so it is not uncomon to find unrestored 500 Nuova and 500D cars.  I think
the same is true for older 600 models.  

Of course, it is probably not worth it to ship a rust-bucket back to
Europe with the possible exception of the Jolly models.

There is both a 500 Jolly (unrestored) and 600 Jolly (restored) listed in
Hemmings Motor News

http://www.hemmings.com/index.cfm/fuseaction/quicksearch.classifieds/keyw
ord/Jolly

Good Luck with your search,
Clifford

try on the following italian newsgroup: it.annunci.usato.autoveicoli

see ya,

--
Stuka (25, 130+, MI)
/F/I/A/T/ barchetta
Momo+OMP
BMC+SuperSprint
bCI member: #359
http://digilander.libero.it/Stuka78/index.html

The message <Yg12c.2918$h_.1867@news-binary.blueyonder.co.uk>
from "Michaela Weller" <mw006d7574@blueyonder.co.uk> contains these words:

Try Simon Lynes 02074987111, or

www.fiat500club.org

ebay!!!
need i say more.