The "battery" is actually a 0.22 Farad capacitor.
The capacitor is for memory backup, and is re-chargeable.
See here :-
http://www.elna-america.com/products/pdf_f...yer/DS_DSKe.pdf
You could TRY and get one from FARNELL (U.K.) or DIGIKEY (U.S.)
Rock Lobster