Get the part # here:
RealOEM.com BMW E70 X5 3.0si Roof moulding/Roof rail

And you could order it here. Just search the part number:
OEM / Performance Parts for Volkswagen, Audi, and BMW - ECS Tuning

Here is a thread they have on XOUTPOST. Just ask them in this thread, and they should get back to you with a price:
ECS Tuning - What do you want from Germany?

Or you might be able to get it direct from your dealer.

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2011 ///X5 M (On Order)

2009 X5 4.8 M Sport Space Gray/Black Fully Loaded

2007 GMC Yukon XL Denali Loaded and Customised