Featured 12V Aux.battery

Best Price and best performance all in one are standard Wheelchair batteries, which are often the same ones used in computer backup power supply batteries. This one is 55amp hour and most all 12v Prius batteries are way less than that: 12 Volt (12V) 55 Amp (55Ah) VRLA AGM Sealed Lead Acid Battery 55 AMPS HIGH CAP | eBay

 

welcome!
start with some dealers, then compare auto parts stores and on line.

 

Depends on the circumstances.

Universal replacement batteries like that might NOT be a good choice.......for someone who really knows nothing about batteries.
The posts might not be the same.......and the posts might be reversed, making it much more likely that it will be hooked up backwards.

Sometimes trying to save a few pennies ends up costing you a lot of dollars.

It is really hard to give a good answer not knowing what the auto repair businesses are like where he lives.