The dealer is given three stickers, one goes on the pre delivery inspection (PDI) repair order (RO), one is placed on the anti theft radio code card and the other can be hidden somewhere or simply thrown out. I used to put them on the glove box out of site so that it could be found when the glove box was removed.
You can try holding down the 1 and 6 key on your radio, this may or may not flash the radio's serial number. The dealer can use the Honda radio's serial number on in.honda.com to get the radio code.
Or if the dealer that sold the car is smart they should have it documented on the PDI RO.