There are hundreds of motorcycle clothing and equipment retailers around the country (and online). Everyone has their favourite, of course. So ask around among other bikers. Otherwise you can find one near you via Yellow Pages or use a search engine.
Most bike shop staff are generally well informed, so ask them to guide you as to what’s best for the type of riding you want to enjoy.
As well as specialist motorcycle clothing and accessories retailers, nearly all the major bike dealers stock a comprehensive range of gear. Here are links to the websites of the main importers in New Zealand, where you can locate dealerships near you.