I thought there would be more options for rotors from the likes of stoptech, but guess not.
If you're doing oem pads, bimmerzone might be a little cheaper
http://www.bimmerzone.com/BMW_Parts_...-BRAKE-M3.html
http://www.hpashop.com/BMW-E9x-M3-OE...Brake-Disc.htm
There was another site that I was looking at before that I think was cheaper, but forgot which.