Razer claims to only have specific authorized retailers for its products. The store locations will vary based on where you live. The Store Locator on their website at [https://www.razer.com/store-finder|https://www.razer.com/store-finder] can be used to find a location near you.
Batteries Plus, Best Buy, and Office Depot all sell batteries (Amazon may be an online option). Still, it may be best to buy a battery from a store that the manufacturer recommends.