Did just that with mine: bought off ebay from BMW Direct together with faceplates. If memory serves me right I also needed a particular type of screw for each that held them in place to the dashboard but didn't come with the order but I managed to pop into my local dealership and pick them up
(see screws here) http://www.z4-forum.com/forum/viewtopic.php?f=7&t=50613
http://www.z4-forum.com/forum/viewtopic.php?f=12&t=7464