New Battery

check Ebay.....there were a few sellers that sold those and extra batteries. I own a set an I pai $25.00 for a pair on ebay new...you actually might find a used set for really cheap on there that would have decent batteries in it.
Unfortunatly I can't remember the voltage of those small batteries. If you can find a battery of the same voltage/type of rechangeable, that will fit into that small area/case, you can just wire it up to be used.

Check Ebay.... they are really cheap on there.... contact one of the sellers to see if they stock them.
You can also check the Xact website or get ahold of one of their reps for recommendations if your really stuck.

Take care