You can get them right from Faller themselves(which I recommend) or buy them through the overpriced Walthers web site....
They have different motors with different speeds as well... I would also get the "Rainbow Gear Package" which basically is 4-6 different gear sizes to adjust the speed control....
Go to the Walthers web site to get the model numbers then either buy through them or go through Faller themselves....
Are you planning an putting these models on Cummons style bases?? If so then the motors will work just perfectly....