Compare Battery Deals
A close price: Campus111 via eBay has it for $14.38 plus $1.19 for shipping.
Of note, add five 4-packs to your cart for an in-cart price of $31.95 with free shipping ($6.39/pack), yielding another price low.
Of note, you can order up to 20 packs for this price, but shipping will then start at $8.99.
Update: It's now available for in-store pickup only.