You can go to the hardware store and buy something similar (12 x 1.25, 40mm long I think?), but it won't be quite right. The threads on the stock bolt are slightly more tapered, allowing it to easily go through the hub but still bolt to the bracket. It is important! You should be able to press the bracket firmly against the hub. If the screw gets stuck in the hub, you'll end up with a very slightly loose mount! Loud brakes and possible danger.