BOC gases are a supplier of dry ice. There are three locations in the Wellington area. Whether they supply in small quantities to private individuals I am not sure, but I suggest you give them a ring and see what they say. They may be able to point you in the direction of a supplier, if they cant provide the quantity you want.
One of their offices is in Lower Hutt on freephone 0800.111.333 and there is a distributor called Zip Plumbing in Adelaide Road on 04.389.0400.
BOC's website 'contact us' page is given below.
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