Centre cap!! Need some help!!!

[xr8 007]

Just got my rims changed by the dealer which was in the agreement when i bought the car that the rims will be replaced as they were totally guttered. Anyways got them changed on the weekend and today when i got home from work noticed the centre cap (rear drivers side) had either fallen off or got stolen. I always have a good look at my car before i take off so i'm leaning more towards the fallen off theory as the grease cap is gone as well.....

anyways where can i get the centre cap for my rims. they have my beloved tickford symbol on them..they are 17inch tickfords..the ones that are found on AUII..how much am i looking at!!

thanks a bunch yet again!!