here is what Jerry said:
Batteries are available on the web, they're 7.4 volt 900 MaH helicopter batteries. However, since there are two in parallel it's very important to have a matched pair as any difference will reduce the lifetime significantly.
There are also slight variations in size and they're a tight fit so it's tricky to get ones that work well, I have lots of useless leftovers that get re-purposed for in-shop instruments and projects. I sell the matched pairs for $100 / pair plus shipping.
They plug in to a little board at the back of the case, no soldering is required. It is however delicate work in a tight place so I'll do the job if you pay the shipping both ways.