. . . also there would be no point having radio codes if all of the radios in a particular model had the same code; they're all different!
There are several online services who can provide the code, such as this one:
http://www.radio-code.co.uk/
The problem with using them, though, is that you need to remove the radio to view its serial number. (That's often far from easy without specialist tools!).
Assuming that the car has its original radio, a main dealer can access the manufacturer's database solely from the chassis number (or registration number) of your car. It's likely to cost around £15 for the service.
Chris