Hey Tomer,
As others have mentioned, car wiper motors and electric window motors are both fantastic for the job. Both operate @ 12V or thereabouts, and both have high torque. As a benefit, wiper motors have dual speeds, fast and slow, so you can choose which works for you. Trawl through some brewing forums if you need any more ideas - some brewers have built motorised mash stirrers and it might give you some idea of how to put together the shaft and blades on the cheap, as well as how to mount it.