What meaning do you put in "rated"? If you are looking for max. efficiency or power, than I don't think you find any motor which will suit you. For the speed, KV rating will have to be very low. To get a low KV motor normally gets bigger in diameter, and then normally it has more power. So in your case - either your motor will be hugely oversized in terms of power (x10 and more), but will achieve the needed Watts at this low speed, or you just look for a motor with a reduction gear. The manufacturers where to look: T-Motor, Hacker, KDE, Maxon motors.