Local mini club:
Minis Manawatu.
There are probably a wider selection, with at least a couple of Estates and Hornets on Trade Me. They're all UK spec (apart from a Aussie-spec S on there at the mo'), some were assembled here from CKD kits. I have a MkII awaiting restoration. There's traders on TradeMe who import parts, also Minibitz in Auckland and of course the UK suppliers ship here.
You might end up becoming familiar with the NZ certifying regime, whereby any significant mods need to be signed off by an engineer, but there's a great guy in Levin (near Palmerston North) and that's down the track anyway.