Use a mod like "Vlads Export Garage" and you will start to see them populate the world.
It requires a new game config and some other mods. I HIGHYLY recommend you download "Add-on Vehicle Spawner" as this mod will eliminate cars disappearing. But also consider that the new ScriptHookV is not released yet so you probably wont see those cars in SP mode until a new patched SHV is released.