I actually saw one on the shelf a long time ago, should have bought it. I have subsequently tried to find one, and it seems to be no where.
Here are some small pumps, kind of pricey.
http://www.lvmshop.co.uk/category.asp?sid=&ls=&id=70
Just curious, why is 12 volts required? Is it because you want to run it through a float switch located in the sump or tank? If so, use the 12v and the float switch to operate a 12v relay. Use the relay to run a standard 110v power head. It will be much less frustrating in the long run.
Paul S