For something of that size and weight you could probably find a battery operated on at a hobby store ( Hobby Lobby, ebay etc.) in the model section. A lot of hobbiest use them for model cars. You can pick them up for under $10. Cover parts with fondant or nestle it in florals. They fit perfectly on a small tier. It works wonders! Hope this helps!
I've alwaye wondered how all thos people on the cake competitions add motors and (larger than what your talking about) make the whole darn thing spin.
If you are wanting your whole cake to spin, you will need to search for a Heavy Duty Motorized turntable. Be sure to know the approx. weight of your cake... All turntables have a weight class the work within as well as speed. Some are slower or faster than other. Some can operate turning clockwise or counter clockwise. The heavy duty ones run a couple hundred dollars. I'm always on the look out for more.
yep, online_annie is right! hobby lobby sells a 7 inch in diameter, mirrored, holds up to 4 pounds, motorized turntable in the model cars aisle. it $20 without a coupon and operates off of C batteries i think. i recently used it in a cake to make the topper spin and it worked perfectly! i coverd the whole thing in one piece of fondant and then layers fondant waves over that. then i just cut a slit b/t the base and mirrored part of the turntable and voila! it spun 
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