Also, classified sections in Ferrari Market Letter, Ferrari Chat, and of course our own Ferrari Life. Finally, I'd be remiss if I didn't mention the Pre-Owned locater on the Ferrari North America website as well.
I've found a few very nice cars listed by private owners on craigslist as well, believe it or not. Since Ferrari's tend to congregate in large metropolitan areas, searching the large cities in each state in the US may turn up a gem. Just a thought.