https://electronics.woot.com/offers/duracell-21-23-12v-specialty-alkaline-battery-4-count-2 4-Pack Duracell 21/23 12V Specialty Alkaline Battery on sale for $2.99. Shipping is free for Amazon Prime Members (must login with your Amazon account and select a shipping address in order for Woot to apply free shipping) or is $6 per order.