HI genuine bmw on ebay. Iv had both and would buy genuine, as the retaining clips are so much stronger and the caps don't rattle around, but about 3 times the price of copies.
Rich
http://www.ebay.co.uk/itm/Genuine-BMW-Alloy-Wheel-Centre-Caps-x4-/281254206539?pt=UK_Cars_Parts_Vehicles_Wheels_tyre_Trims_Trims_ET&fits=Car+Make%3ABMW&hash=item417c0e9c4b