Reloaders supplies has had them in the past, (their website doesnt seem to be working at the moment). Have you tried the NZ Miroku importers (Outdoor Sports NZ)? Would most likely have to order through your local store if they did.
Alternatevely I'm pretty sure the Japanese made Browning B25 series of shotguns use the same firing pins as they are manufactured by Miroku so those suggested by Quackers should work as well.
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