I think most folks just use the typical 7AH field box battery:

http://www3.towerhobbies.com/cgi-bin...?&I=LXL370&P=0

If you want something bigger, make sure it is a deep cycle battery. Your typical car battery is not designed for repeated full discharge. They are meant for starting the car and then charging right back up. The lead plates in a deep cycle battery are thicker, and they are designed to discharge most of the way and then be recharged.

The little battery in my field box is usually good for a couple months worth of starting before it needs to be recharged.

Good luck