FYI: Amazon sells various versions of the BMW center caps including BMW OEM models. There are some very nice ones for as little as $35 for a set of four.
You do have to be sure of the wheel bore size since they come in 65, 68 and 70mm sizes.
See:
BMW Wheel Caps
Also available are some beautifully made domed-alloy
stick-on versions that can be applied over the usual aftermarket cap that is supplied with the wheels.
Amazon calls these "decals" but they are actually very nicely made, adhesive-backed metal cap "covers" and work fine.
Also available is the "too cool for snow tires", "
M" version (as a complete cap).