Well if you're not too picky about the shape, Supercheap in Sydney sells these rounded top ones (same shape as the silver one below whose picture I stole off eBay) in matte black for a couple of bucks.
I used those cheapy matte black ones when I first got my wheels in fear the originals might be pinched. But me being me, I couldn't stand the shape of them so I chucked them out and put the original chrome ones back on.
But yeah, the most annoying thing in the world getting these stolen. Lost 2 sets on the previous car and 1 set on this one (when I had the shadow chrome HP Design wheels). I actually caught a little sh*t in the act of stealing my caps last year, and was going to introduce my foot to his arse, but I didn't want to be jailed for assaulting a minor.
