Predator Free New Zealand has heaps of information about what traps to use and where to buy them. The information includes knowing which predators you are targeting and which ones you aren’t and how to use the traps.
There’s a great table which outlines the best type of trap to use depending on the type of predator you are trying to target e.g. rats, stoats, ferrets and possums. The information is based on the Department of Conservation’s best practice guidelines