Where can I buy battery for it

Unless you can find an electronics repair place, this may require diy dissasembly to locate the battery, substitute it with a temporary battery/power supply to ensure this wireless speaker system still operates before buying a replacement battery pack. Once you determine battery size, capacity, and voltage, you are free to choose any battery pack from online sources. You may consider larger capacity too; recharging may take a little longer but you also have longer playing time between charging.