How do I get a new battery for my iPhone 6?

To get your battery replaced:

• Visit the Contact Apple Support page.
• Click on "See Your Products," and then, sign in with your Apple ID.
• Select your iPhone, and then, click on "Battery, Power, and Charging."
• Click on "Battery Replacement."
• Finally decide if you will "Bring in for Repair" or "Send in for Repair."

Hello,

Take a look at this article:

iPhone Battery & Power - Official Apple Support

Can I still buy a battery for my I-Phone6 for $29??

yes you'll be able to get a replacement battery at that price til December 31, 2018