Given the geology I have read about in Kiwiland, you ought to have both agate and obsidian somewhere on your islands. Both are found where there are hot springs and volcanoes. Obsidian is super easy to knap yourself into excellent sparking flints but rarely lasts more than a couple shots due to the brittleness of the stone so you must carry several per outing. Agate requires more preparation including heat treating but ultimately can be knapped yourself. You have to be patient with agate since not all agate sparks well (I have no clue why). I have even found a few especially hard jaspers that will spark on steel. If you happen to have a tile cutter left over from a home improvement project, you can even do a little rock cutting on small pieces yourself without having to buy an expensive rock saw. I will bet the right stones are out there if you develop a rockhound's eye for looking down where you walk.