I'm one car from 100% the game on PS3. I've looked all the usual places that people say it spawns(Broadway etc.), but I haven't seen one at all.

Does anyone know where and when there is one that spawns stationary?(parkinglot, during a mission etc)

Here you go: 
(Images found/belong to the L.A. Noire Wiki) 
  
Car can also be found in this area: Olive St, between 9th and 8th street.

Oh, man, can't believe you're missing THIS one. I saw SO MANY of those near central police station.

I finished my collection yesterday, and here's a way to help you that will seem crazy, but I swear it works. Go to the Showroom, search the car you're missing (only works for 2 and 4-doors vehicles), press A or X over it. Now go back to main menu, cases, Homicide Desk, Free Roam.

As you start, the vehicle will pass by near you, or it will be parked across the street. I swear, I got every 2 and 4-door cars I was missing that way.

This is the only thing I have left to do, get all the cars. I got about 50 of them naturally but since it's the last thing I would like to do it. How long does it take?

Oh, man, can't believe you're missing THIS one. I saw SO MANY of those near central police station.

I finished my collection yesterday, and here's a way to help you that will seem crazy, but I swear it works. Go to the Showroom, search the car you're missing (only works for 2 and 4-doors vehicles), press A or X over it. Now go back to main menu, cases, Homicide Desk, Free Roam.

As you start, the vehicle will pass by near you, or it will be parked across the street. I swear, I got every 2 and 4-door cars I was missing that way.

@Scapegoat: It does work for undiscovered cars.