For my 595 I have used Partsworld (Stoneacre Group) and TMC, both are really prompt on the delivery front, TMC do charge delivery and they both often have sales on.
So if it is someting you want and it's not urgently needed, bookmark the page and go back to it once in a while.
Partsworld-uk is the online parts and accessories shop from Stoneacre motor group. We specialise in supplying genuine Fiat, Abarth, Alfa Romeo, Jeep & more parts, accessories & merchandise. Take a look at our range today!
www.partsworld-uk.com
For my Cinquecento, I have used Ricambio, they send out items nice and quick, they charge for postage and you'll get a couple Fiat 500 shaped chocolates thrown in with your items. They cater for all older Fiats, but parts for Cinquecentos, especially things like RHD headlamps (Sporting, driver's side) and Sporting rear light clusters are getting hard to find now.
Otherwise it's a case of looking on ebay and waiting for something to come up and pray that the part I need isn't coming from Italy.... Where shipping and taxes take the proverbial.
A friend of mine with a few Alfas uses Alfa Shop and says they are really good, they also sell Fiat parts, so I may try them out in the future.