how do i recover my car radio code?

Hello everyone! I wonder if anyone would help me? my car battery died outside school when picking up my kids the other week and ever since then i have had no music. I have been in touch with my dealer who wants £25 for the code which I am not happy to pay. So i was wondering if anyone could help me please?? I am not sure if I know where to look as far as serial numbers are concerned, so please someone guide me! It aint gud with no chili peppers playing, as all I can hear now is my kids bickering!!!