Where can I buy replaceable batteries?

I have this toilet which my mother-in-law gifted me on our second wedding anniversary. I, however, would like to have several spare batteries for use when I leave for the countryside and have the kids remain at home. How much do they go for and where can I buy them? It looks like the nearest shopping centres in my areas don’t stock them. Also, what is the battery life until they completely fail to work? Is there any special handling?