How do you find the lost remote locking key of a car?

you cant,if you happen to find one exactly like yours i.e. somones lost keys,nearest dealership carries extras,who knows where youll find one,usually you can put your key in the ignition turn it on then off eight times in succession without starting it you should hear your doors lock,then you press the unlock button on the foreign keypad then it should be programmed.god only knows how many times ive had to do this on my Ford but im not exactly sure which makes or models this works on try it out,if not,your gonna have to pay out some bread at a dealership for a new one