Why are you looking for a battery? The life on this phone is simply ridiculous! I can easily go 2 days without charging, it's usually above 40% by the end of the day, and that's when travelling and playing games, etc. Is your battery damaged? If it is, just take it in and get it replaced since you phone is new.
If you just want to get one to have one, that's not a good idea. Lithium-Ion batteries begin loosing life expectancy as soon as they're manufactured. Even if it's just sitting, not being used, it will slowly die away. You should never buy a second battery until you actually need it.