Where do you recommend getting a new battery for my iPhone?

At the Apple Genius Bar or a certified Apple repair facility such as Best Buy.

Do not use a third-party repair shop. It may be cheaper at first, but you will come to regret it. Third-party repair shops cannot purchase genuine Apple parts so you will get an inferior battery that most likely won't have the capacity of an Apple battery and will cause problems down the road. Also if your phone ever needs repair, Apple won't touch it if they see it's been tampered with by a non-authorized service center.

Repair Provider Verification - Official Apple Support

You can find a list of authorized Apple service shops in your area by using this link......Find Locations

$69 (U.S)