I've been thinking about getting a few of these....
http://www3.towerhobbies.com/cgi-bin...&I=LXVRH2&P=ML
6v - 1600 mah and just under $15...... If you don't have a charger, you may need one as I don't think you can trickle charge batteries over 700 mah. I think Tower also has an inexpensive Nicd / Nimh peak charger for less than $30..