my poor milage and need battery

What is your question?

Hi everybody.i have 2003 prius.Recently it giving trouble,i dont know. just it from earlier it just low milage.just 23 milage per gallon.May be i dont know good yet how to drive in hybrid drive or starting.Anybody can help me about that.Anybody know how i get 12 volt auxilary battery that will fit my car.Bec i try lot of places i didn't find battery.Today i went auto zone but they dont have it.plz anybody know those issue messege me.Thanks