Replacement battery for my iPhone 6s

Yes, you can. :) If you want an original battery (which you probably should), I would recommend an Apple Store, if you have one nearby, or an Apple Authorised Reseller. Especially if your phone is under warranty. Not sure about the prices though. They might be country depended. But you can always contact the store itself and ask.

Apple doesn't sell or distribute iPhones original spare parts to anybody, except to their Apple Stores or Authorized Service Providers.

Regards

Giulio