You don't say whether it's a factory fitted one or not?
Try under either front seat, try behind the glove box, try under the spare wheel in the boot and any side compartments either side of the boot - even try looking behind a panel to see if he's fitted it in a disused space for that model Failing all that look at the manual that came with the car.
Failing that get the Vehicle registration form and phone the previous owner, if he's not listed write to him. I'm very suprised that the dealer couldn't help though - I wouldn't be going back there !! Good Luck !!