Image courtesy of Adafruit

CR1220 coin cell batteries output about 3V and have a capacity of 40mAh. That means that you can expect one to supply 40mA of current 1 hour, or 20mA of current for 2 hours, etc, etc. These batteries are often used by real-time clock chips to keep them ticking even when their main power supply is disconnected.

Buy a CR1220 coin cell battery from Amazon, Adafruit, or Newark.