Battery type, cost, retailers

The battery type is LR44/AG13. The LR type is an 'Alkaline' type battery. Each battery manufacturer uses different numbers to identify the same type, do not be concerned about the exact identifier if a retailer offers the correct size. For reference, wikipedia has a list of common identifiers for this battery type.

A pair of good quality LR44/AG13 type batteries from a major retailer will cost around NZ $5.

Warehouse Stationary (Energizer)
Mitre10 (Energizer)

For larger quantities at reasonable prices contact a trade supplier like RS Components.

You could also consider using SR44 equivalents. The SR type is a 'Silver Oxide' type battery recommended for high drain devices. These are more expensive than the standard type.

Whatever the type, please ensure the battery you choose is mercury free. The SR type is more likely to contain mercury unless specified otherwise.

TradeMe lists plenty of LR44 and SR44 type batteries in various brands at very low prices. Some very good deals, buyer beware.