The required battery is really generic - you need something between 3V and 5V and solder it to the two points. I'm pretty sure that some energy source like that will be around for the next few decades, no matter where on the planet you are.
The needed current is really small, so I guess even a few lemons with copper and aluminium nails in them will do the job until you find a battery. The OKI chip starts running at 2.4V, and the lower end you get out of a lemon is 0.7V, so tell your local supermarket to set four lemons aside for june 2014 :-)
Jens Schönfeld