Some of the tracks in NZ designated "Great Walks" require booking. You can get info online
here.
If you haven't already, check out the
NZ Tramper forum for more information on these and other tracks in NZ.
DOC offices & outdoor shops in NZ sell topos, or you can get more info on the hard and soft versions of NZ topos from
LINZ.
Also, if you don't want to commit to a booking for your Overland track walk, you can do the Cradle Mountain area as a day walk, then walk into the mid-section of the Overland Track via Walls of Jerusalem, all without booking - though you will still need to pay for the parks entry pass and of course organise transport to the start of the tracks.