Online: You can buy Transported from a number of online booksellers, including Fishpond, Whitcoulls and New Zealand Books Abroad (which I recommend for overseas customers). A quick Google search will turn up others.
In person: You can buy Transported from the bookshops listed below. I'm sure there are others - if you find Transported in a bookshop not listed here, please let me know by an email to email@example.com. If you go to a bookshop that doesn't have Transported in stock, please encourage them to (re-)order it.
The following Whitcoulls stores should stock Transported: Bennetts Pier, Auckland Domestic Airport Terminal [signed copies available], Bennetts on Broadway, Botany Downs, Cashel Street, Centerplace, Chartwell, Corner [not actually sure where this is!], Courtenay Place, Dunedin (George Street), Dunedin (Meridian Centre), Galleria, Henderson, Invercargill, Lambton Quay, Milford, Mt Maunganui, Nelson, New Lynn, New Plymouth, Northlands, Plaza Palmerston, Queensgate (Lower Hutt), Riccarton, St Lukes, Taupo, Wellington Domestic Airport Terminal [signed copies available], Whangarei
Auckland: Unity, Borders, Time Out, Daniel's.
New Plymouth: Wadsworths
Wellington: Unity [signed copies available], Dymocks [signed copies available], Parsons, Borders, Johnsonville Paper Plus
Nelson: Page & Blackmore
Christchurch: Borders, Madras Cafe Books, Arts Centre Bookshop
Dunedin: University Bookshop
... plus other Paper Plus outlets throughout the country.