Ballantynes is a fairly big department store on Cashel St. in the city centre. The nice things is that they offer no GST (sales tax) on all your purchases, if you're overseas and flying out of Christchurch, simply buy and then pick up your items at the airport when you leave. 12.5% off isn't that bad. Mind you, Ballantynes is slightly more expensive than other stores, but without the GST you will still be saving money buying here, rather than at the souvenir stores elsewhere.
What to buy: Woolen clothes, esp. with possum fibres.