My Astra Sports Tourer is fitted with 18in alloy wheels, I've been trying to purchase a spare wheel from Vauxhall, but their customer care department say I can't have one as my car is fitted with 18in wheels, I can only have the tyre repair kit supplied with the car.

I have proved that the wheels fitted to my car do fit in the spare wheel well in the boot area and the the boot floor fits back perfectly flat. As I travel abroad with the car I really do want to carry a spare wheel, where can I purchase a correct steel or alloy wheel to act as a spare for my car?