As Wolfgang noted, as a real theft deterrent, door locks are pretty useless. But really, people don't usually steal odd, one off, easily traced toy cars- they steal Honda Accords.
However, door locks are quite effective at keeping REALLY curious people from feeling free to open a car up and have a looksie. It happens.
Intermeccanica has outside door handles with lock cylinders standard on their cars, but these are a full-on custom piece- they make them in Rivendell.
A less expensive (and maybe even better) idea would be to install a remote lock kit. If you do it this way, there's no expensive hardware and you can add an alarm with some motion detectors, etc. to add some actual theft deterrent, if that's a consideration.