I got my battery from here:http://www.newmp3technology.com/404-battery.htm
they are reliable but more expensive, though they also sell on ebay occasionally.
It is easy to replace since has a small plastic connector, so no soldering needed.
Take out the 4 small screws on the sides, put face down, and CAREFULLY lift out the back plate. The battery is usually stuck to the pcb underneath with double sided tape. Slowly lift the battery away from the tape without pressure on the pcb or the wires. It can then be flipped away from the unit still on the wires. Now you can lift the edge of the hard drive to see the battery wires connector and squeeze and pull it with some tweezers. Connect new battery, re-stick to pcb and confirm hard drive is flat down. Screw up back panel and make sure it turns on. Recharge until full before you start using it for any length of time.
Good luck :-)