The Varta part number for the 3.6V 70mAh Ni-MH battery is V80H.
It is exactly the same size and shape as the original "barrel" battery.
Most industrial electronics supplies have them such as Farnell and RS.
I also have a stock of them here, NZ$14 each (about 7 Euro).
Not sure about international shipping though, as they may be considered dangerous good??
