The AG13 battery is a popular 1.5V button cell battery used in many small electronics, watches, calculators, and toys. 

Best Deal
AG13 Batteries on Amazon

Equivalent Batteries
AG13 batteries are compatible with other 1.5V button cell batteries.  They come in many names with the most common being LR44, LR1154, SR44, A76, D303, D357, 303, and 357 batteries.
AG13 Equivalent List

There are so many places to buy these, but what is a reasonable price or a good deal?

Over PricedAround $3.33 to $4.99 per battery + tax (Radio Shack) 
Retail PriceAround $1.89 per battery + tax (Walmart)
Great Price$0.30 per battery. 84% cheaper than Walmart! 

Usually to get the good deals, you need to buy the batteries in packs of 10 or more, but one pack of 10 batteries cost less than a pack of 3 at Walmart. 

AG10 Battery LR44 Battery Equivalent