The battery is a No. 3710 - 3V - 2R10.  The ones I have are 'Varta',
distributed in Great Britain by Varta Ltd, Cropmead Industrial Estate,
Crewkerne, Somerset TA 18 7HQ, and in France by Varta S.A., 92403
Courbevoie.  I try to pick up one or two batteries any time I am in
Europe, but I would expect you could have somebody send you one.

Also, you could disassemble the battery carriage, by taking off
the ends (they are screwed on) and fabricating a new carriage to
accommodate two size 'C' cells, and reinstalling the original ends
with the contacts.  I have done this, and it works well.  Keep the
old wooden carriage, so that it could be reinstalled to its original
condition if desired.

Dan Wilson
North Carolina