misterjames,
Crossed drilled rotors while used in the racing world and on motorcycles are not necessarily the best for your 850. In fact, you are more prone to warping with cross-drilled rotors. A fact that for example IPD has recognized and
now sell etched rotors instead. FWIW, stick with OEM rotors and consider alternative pads for better performance and less dust. I have OEM rotors with "Pagid" pads on my 850. "Pagid" are made in Germany and are OEM on BMWs. They deliver the same performance as OEM (which is good) but generate less dust at the same time don't squeal. As far as purchasing on-line...you can purchase anything and everything on the internet...