If your ride is the correct one from a quick search i just did if you have the original wheels they are vt series 1 berlina mag wheels and your best bet if holden doesn't have them would probably be ebay
For What I can remember, The Centre Caps for the VT Olympic Edition are the same as the Series 1 Berlina Rims Centre Caps.
From wikipedia the picture of the olympic series vt uses these centre caps https://upload.wikimedia.org/wikipe...II)_Olympic_Edition_sedan_(2009-07-05)_01.jpg https://www.ebay.com.au/itm/GENUINE...560785&hash=item41e7b4d8cd:g:WIgAAOSwDQpbTWx0
You can find them on the side of the road somewhere in the vicinity of Geelong because I've lost two of mine in the last 6 weeks or so.
Ideally, it would be best to post a picture of the rims you currently have fitted....From there, it makes it easier to determine the best place to get them
This style is as common as day turns to night, so any holden wrecker and even tyre shops have these laying around.... If going this style, I'd recommend finding the metal ones