You can get quality 6C 7.2V Nimh Packs from Hong Kong on Ebay very very cheap.  I've bought several of them in the last year and I've never had a single problem with any of them.  They charge and run better than batteries I've owned costing 3 or 4 times as much.  There's people who buy them 10 or 20 at a time.

You can get a 5300mah for $12. . . . 3600mah for $8 . . . . plus around $8 shipping.  Just search "7.2V Nimh Tamiya"</p>