I wanted one of those models, really bad, when I was 9 or 10 years old. I was lucky enough to get the big Cat wheel loader, but after that mom and dad said 'you already have one, why do you need another?'.
Well, it sounds like you'll need some gear motors. Check these guys out.
http://www.servocity.com/html/motors___accessories.html
There are many other places you can find them too, even in smaller sizes.
Here's another good place.
http://www.robotcombat.com/products/motors_main.html
And one more -
http://www.robotshop.com/robot-parts.html
I have only ever ordered from Servocity, so I can't speak for the others, but I have always had good experiences with Servocity. Great service.