AG13 batteries are a popular 1.5V button cell battery.  They are the most popular alkaline button batteries due to having the highest capacity in their family of batteries. 

Best Deal AG13 10 Pack $1.99 with Free Shipping

AG13 Batteries on Amazon

Single AG13 button cell battery

Battery Specifications
Voltage: 1.5V
Diameter: 11.6mm
Height: 5.4mm
Chemistry: Alkaline
Capacity: 128 mAh

Battery Equivalents
LR44, L1154, SR44, SG13, LR154, A76, S76, EPX76, 157, 303, 357.

Where to Buy
You can find these button batteries at your local department store.  Duracell and Energizer will call them 303 or 357 batteries.

Battery Cross Reference