Dunlop Foam in East Tamaki. They prefer to deal with accounts rather than cash purchases - sometimes they allow cash purchases and sometimes they just say no.
NZLARPS is in the process of getting an account but not quite there. The form is lying around my house - given that we buy a sheet every few years it hasn’t seemed urgent but if someone wants foam now we can finish forming the account and buy on your behalf.
I have a 1x2m sheet of PE45 foam, which is denser than the stuff we use for sword blades and probably suitable for shields. It belongs to NZLARPS, if anyone wants it they can have it for $46.83 as I don’t think the society needs it immediately. Or if you’re a project you could put it in your budget.