Only Apple or an Apple authorized service provider have access to new iPhone batteries to replace them with.
Apple does not sell iPhone batteries, and there is no sales mechanism for non authorized repair shops to get access to OEM parts.
Any battery used by a non Authorized repair shop, would at best be salvaged from a different iPhone, a fake, or a 3rd party option that may not meet the standards required, and would have no guarantee of working very long.
If you want to retain your warranty, and be sure you are getting an authentic battery, only replace it at an Apple Store or Authorized service provider.
Currently Apple is charging $29usd, for battery replacement of out of warranty phones, or $0 for iPhones still in warranty. So its hard to imagine "any old electronics store" would have a better price.
iPhone Service Pricing - Apple Support