Apple will replace your iPhone 6, 6s, or 7 battery for $29 even if it's not being throttled
Replace the Battery of iPhone SE/6S Plus/6S/6 Plus/6/5S/5C/5- Dr.Fone
Apple Repair iPhone 6 Battery Replacement Service
Amazon.com: EMNT 5500 mAh Battery Compatible with iPhone 6s High Capacity A1633/A1688/A1700 Battery Replacement 【New Version】 with Repair Tool Kits and Instructions【2 Years Warranty】.! : Cell Phones & Accessories