You can buy the BMW rails and attachment feet, but they are at BMW prices. I like the Thule load bars, they are robust and easy to attach to (rectangular). The Thule Crossroad 450 attachment feet work with many different rail profiles, they have a rubber strap that cinches around the rail, tightens securely, and can be locked in place. They fit perfectly on the X5 rails.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue