http://www.ebay.co.uk/itm/4-x-BMW-Alloy-Wheel-Centre-Caps-Badge-68mm-Fits-1-3-5-7-Series-/141735762647?hash=item21001c1ad7
Can recommend these. They have the same look as a bonnet badge with a thick plastic covering, unlike most which are basically a bit of cheap tin plate painted.