I am not really giving you a solution to the problem, but some pointers:
These are the 16 inch rims / 4.8 pre-face lift.
I think that the 4.5 and the 2005/4.8 post face lift Center hubs caps are different.
So I guess those once (yours) are quite scares.
What I would suggest is to take a photo of one of the remaining center hubs inside and outside and add it to your post.
David Volschenk has a 2003/4 4.8 which had the same rims. However I doubt that the rims or center-caps are still available. Nevertheless give him a call on 0834991588.
Than there is Johns Engine and Gearbox in Boxburg which is currently stripping a 98 4.5 . Wheels and center caps are still there , however they might want to sell the rims and center caps together ? Should the 4.5 Center caps fit than this could be an option.
Tel. 011 892 4746
Third option is to find something on this forum.
The last and most expensive options is obviously the dealers.
After a long search I finally gave up and bought a replacement cap for mine at the beginning of this year................ was around 950 Rand