You'll need the serial number off the radio first, this'll be located on a sticker on the radio body which will usually mean pulling it out of the dashboard.
Once you have a serial number then someone like
Can provide a code for a small fee
If you check the handbook first though as often the radio code is on a sticker inside the front cover or inside the audio manual.
It would also be worth contacting the local dealership as they often carry the codes for recently built cars ( last 10 years)