Can anyone advise me if I can buy a Dual sim adapter for a Samsung S3 in NZ

I would like to be able to run Telecom as normal but sometimes work in an area where the only cover is Vodafone 

We are going to be in Chch & Auck soon would prefer there if possible I like buying from a shop so I can be shown through how to etc as well as make sure I have the correct one

I have tried google and trade me but only options seem to be online 

Many Thanks Pete