hi mate, if you have a look at the front of your service book it should tell you who the selling garage was when it was brand new. If you then contact them and give them the reg.no. and possibly the chassis number then they should be able to tell you!!
how old is your peugeot and what model/year is it.
if the dealer doesn't have the code on file, providing it is the origional radio fitted to the vehicle they should be able to obtain the code from Peugeot themselves, though there will be a charge for this.
hope this helps, Neil.....