Find the correct PN on realoem.com : input the last seven digits of your VIN ; click on "browse parts" then look under "Vehicle electrical system/ Switch" ; find the correct PN.
You can get then get the part at your dealer at possibly a fairly high cost or go on getbmwparts.com at a lesser cost but you have to pay for the delivery.
If you are OK with used parts then go ebay but beware, they all say they have a compatible part but that is often not true : there is 2 types of switch depending on your build date.
Z3 2001 3.0L 5-speeds,Topaz Blue/Sport+Premium package/Stong-Strut/Bilstein shocks/Rogue Engineering RSM/Ireland Engineering rear sub-frame bushing/CDV delete/Conforti CAI/Jim Conforti Shark/DRL module/European Wiper Module/Pioneer AVIC-N4 GPS-Receiver+XM Radio+iPod/iPhone interface+Bluetooth+12CD Charger/ASA-AR1 18" wheels+Michelin PSS/Bi-Xenon upgrade.
Latest mod : 3.73 diff. ... WOW
X3 2006 3.0L Automatic, Silver Gray, all original ... so far ...