Find the following cars by searching the corresponding locations:
Attrazione car location
The Attrazione car occasionally spawns in the parking lot next to the Demolition Derby activity icon. It spawns in the fourth parking space to the left of the activity-start facing the building. Many tries may be required before it appears. However, certain cars appear more frequently if you are in a car of a similar class.
Nelson car location
Go to the front door of Price's crib to see the exit (gate). Next door to that will be a house with a Nelson car outside (normally done up).
Zenith car location
The Zenith car occasionally spawns in the parking lot of the National News Center building in the Union Square Region of the Downtown District. To get there, find the drawbridge. It is the second bridge from the left on the map. Follow the bridge north to reach a four-way intersection. Keep going straight. You will eventually get to a T-intersection. The parking lot should be on your right, and is very small. The Zenith will probably require a few attempts before it spawns. However, certain cars appear more frequently if you are in a car of a similar class.