Replacing missing cap

Assuming, from your rig pic, that it's for a Toyota, you try a either a Toyota dealer or a wrecker.

Have you tried Toyota genuine?
Maybe a bit $$$

I've had a bit of stuff from this mob, northern Adelaide suburbs - they carry common centre caps - could be worth a try.
Just Wrecking Toyota

ebay quite often has wheel centres for different models available
just keep looking once in a while

http://shop.ebay.com.au/i.html?_nkw=toyota+wheel+centre&_sacat=0&_dmpt=AU_Car_Parts_Accessories&_odkw=toyota+alloy+rim&_osacat=0&bkBtn=&_trksid=p3286.m270.l1313


found a few here take your pick

Try Tony's Auto Wreckers in Midvale I know he is wrecking a 40th aniversary '97 Toyota

Cheers
Pop

Hello Jugga ...
Try placing a notice on this web site: Trader/Parts and Spares/Wanted Items

I did this when I was searching for a spare rim and tyre and I got a very quick response from a nearby Forumite.

Cheers,
Netnut