When can I purchase a battery for $29 for my I phone 6+?

https://www.apple.com/iphone-battery-and-performance/ - December 28, 2017 A Message to Our Customers about iPhone Batteries and Performance

iPhone Battery and Performance - https://support.apple.com/HT208387 - about power management, battery life

Checking battery health using Console - https://discussions.apple.com/thread/8308874?answerId=33120652022#33120652022

iPhone Battery & Power Repair - https://support.apple.com/iphone/repair/battery-power - battery replacement and cost

iPhone 6s Program for Unexpected Shutdown Issues - https://www.apple.com/support/iphone6s-unexpectedshutdown/

How to find your nearest Apple Authorized Service Provider (AASP) or Apple Authorized Distributor (AAD) - https://locate.apple.com/country

Genius Bar appointment - https://www.apple.com/retail/geniusbar/

iPhone Battery & Power Repair - https://support.apple.com/iphone/repair/battery-power - battery replacement and cost --> "*Through December 31, 2018, the out-of-warranty battery service fee is $29 for all eligible iPhone 6 or later models. Battery service at $29 may be limited to one repair per iPhone."

Is that what you're looking for? Otherwise my answer to your asking, "When can I buy" is, "Right now and until the end of December".