EDIT: METHOD CLOSED - UNRELIABLE.
1) Park near a police station in free roam (tested in private match)
2) Start a job (glitch tested using job requested from Gerald)
3) Cruisers should appear parked normally like they would in story mode
4) Take desired vehicle and follow usual instructions for storing/insuring in garage (http://gtaforums.com...cars-in-garage/)
5) Enjoy the best car in the entire game
Credit goes to forum user SAYAGO for perfecting this method of obtaining these cruisers. My original post appears to be correct in that they spawn at the police station in East LS during the mission "Flood In The LS River," but looking back I remember starting the mission close to the police station. Hence why they appeared then but never again... until now! You must be near the police station for them to spawn.
SAYAGO also discovered once the cruiser is obtained, returning to a police station with it will trigger the car spawns again. Unlimited cruisers for you and your buddies.