in U.S.-check midwest bayliss, they have almost everything, may pay a bit more but they have good items in various states depending on what you want to pay- make sure you know what you want before you order (there are so many things that look right, but aren't "correct")
IN U.S.-check autoricambi
GLOBAL-check ebay -usually cheap stuff, but iffy on condition sometimes
IN U.S.-there was a ferrari parts place I picked some things up at, they made repops of things, but I can't find their website anymore. I will see what i can find.
IN U.S.- there is a store that deals in a lot of repop fiat, lancia, ferrari repop stuff, I need to look up their name though.

Honestly, you have to search for a lot of things in multitudes of places, but you can also look here on this site not only for parts, but for confirmation on which ones are the "correct" ones to buy. If you want original look, the Stratos has a truly corporate parts bin approach. Fiat, Lancia, Ferrari, De Thomaso, etc. all used parts that were on our little cars.