I would be very surprised if this was pre wired for cd changer. The easy way to check is to remove your radio and check the top plug. (the longest one.) if you just have 2 wires then it's not pre wired if it has 5 then it is.
You may find some on ebay, other than that it will probably be main dealer I'm afraid. You will also need the power loom aswell as the fibre optic cable. This supplies live, earth and wake up signal to the cd changer and connects staight to back of the head unit.