Have you tried RatShack or a hobbystore? I bet you could easily find a 4.8 volt (4 x 1.2V cells) battery pack intended for some sort of small eletrical gadget. Then just add a 5V DC transformer (walll wart) to charge it. You could even build your own from loose 1.2volt cells.

-Mike
_________________________
EmpMenuX - ext3 filesystem - Empeg iTunes integration