The SR44 battery is used in various small electronics including but not limited to watches, calculators, hearing aids, laser pointers, toys, and tools.

Best Deal
SR44 Batteries on Amazon
or use the alkaline equivalent from BatteryBlowout.com

SR44 Button Cell Battery

Specifications

TypeSilver-Oxide Alkaline
Voltage1.55 V
SizeRound 11.6mm x 5.4mm
Capacity150 mAh to 200 mAh

Where to Buy
SR44 batteries are made by the major battery brands like Energizer and Duracell and can be found in many department or online for about $5 to $7 for a 3 pack of batteries. 

However, to save money there are cheaper equivalent batteries that will work just fine.  The cheaper compatible batteries are manganese dioxide alkaline batteries and are most often labeled as AG13 or LR44 batteries which basically the same dimensions and voltage.  They can be bought for $0.30 a piece.

This battery pack of ten AG13 batteries costs less than three dollars. Using AG13 batteries can save you a lot of money.

LR1130 Battery CR927 Battery