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
Saying "can't get anywhere' is not a very technical description of the problem you are having...