im not sure if you can off holden themselves but you can buy 15x7 ceptors from holmart Wheel & Tyre Package ..they come withsame tires as cops use (225/60) but theyre ****in expensive!... although the tyres themselves are nearly $200 each so i guess if you look at it like that its not too bad. 15x6 tires (205's usually) will go on 15x7s but it looks pretty wierd cos theyre too thin for em.
just have a look around on ebay/tradingpost/forums etc they always pop up. calaisturbo is good for 15x7s usually