If you click on the link at the top of this page that says "parts dealers" you'll see links to Formula 4x4 and 4x4 accessories and tyres, both of which do tyre covers. There are quite a lot of places that sell them, and its worth shopping around cos prices can vary. Try looking in the back of a 4x4 mag, or search for wheel covers on google or any search engine.

Also worth trying is searching on ebay - sometimes they are on there (brand new), altho not always cheap.

Hope that helps,
Nat