Buy a new iPhone 6 plus

iPhone 6 was discontinued some time ago. So you'd need to look for a used one if that's really the phone you want.

Before you buy any used iPhone, read this link so you don't get scammed --> The All Too Common SAD Reality of Buying … - Apple Community

The iPhone 6 hasn’t been sold for over 3 years, thus the chances of finding a new unit are non existent.