You've been asking this for a long time. 20" factory (OEM) wheels are the cheapest from Morristown BMW. Check the "Accessories Forum sponsored by Morristown BMW" for their deal on it. I doubt you will find a cheaper price if you want to stay factory and not replica.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr