Since BMWs have high-performance brakes, you can't simply just replace the pad. With other cars, you can replace the pad maybe two or three times before it's time to change the disc, but with a BMW since the brakes are so powerful you need to change everything, the disc and the pad. That's why BMWs are expensive to maintain. If I were you I'd forget about aftermarket parts, my experience with aftermarket has been a shitty one, even when buying the "same" brakes from shops that claim to sell the exact same brake that comes OEM (original equipment manufacturer) with the car. Just go to your BMW dealership and ask for brakes and pads.