Yes, I can buy a cheap battery off the line and do it myself, it's too easy. But it's just impossible to buy a genuine apple battery. Apple charges NZD 116 and 132 in US and China respectively, through their official service channel.
But there is NO official Apple service channel in NZ. The apple website only guides you to those agents like iStore and Fonefix, and says "cost and terms may vary". https://locate.apple.com/nz/en/service/?pt=3&lat=-36.7988517&lon=174.7041225&address=0626
So, which one of those agents do you guys recommend? Is it really important not to use 3rd party battery?