You can call your local BMW dealers or try
GRomani@bmwofnewport.com. They give members of E60.com a 25% discount
http://www.bmwofnewport.com/staff.html
For 20" -
Front & Rear alloy wheels: 20x8.5" ET33 - discounted list is $479 each
Wheel part number: BMW 36116792594
If staggered:
Rear alloy wheels: 20x9" ET44 - discounted list is $486 each
Wheel part number: BMW 36116792595
Alloy wheel finish type: BMW Silver