Here are some links that I have found pretty much on my own, through long nights in front of my computer, talking to folks, and networking. And it's all FREE!!
2131-D Delaware Avenue Santa Cruz, CA 95060 (831)459-8128
The God of the 1500 (and other Fiats as well) is Chris Obert. If Chris can't find it, they didn't make it. I put him first because after a LONG search for parts suppliers, when I first half-heartedly asked him if his company happened to deal with the 1500 and fully expected a 'No, sure don't", he said 'Yup, sure DO...What do you need...?" I highly suggest getting in touch with him first if you're looking for something.
Lotsa good help here, especially in the Forum section, although I think they should add a search function to help you find topics pertaining to whatever problem you're experiencing. Cool to browse through, though.
They sell used parts for the 1500 and most every other Fiat as well.
International Auto Parts in Charlottesville, VA
Does NOT...REPEAT NOT!!... stock ANYTHING for the 1500, but I'm putting them in here because they seem to be a pretty sharp outfit. They're a good resource for all you 124 and Alfa owners out there, and they have great looking keychains. (That's where I got mine :)
This site is run by a guy named Henk Hoogenraad, in Holland. He's got a couple of good links to various 1500-related things in Europe, various types of 1500's, plus a picture gallery.
This site has some great photos of the 1500; good for a reference.