Many of the shops do not sell a lot of their plants with names on, just "Bunched plants" etc. Stock comes and goes all the time too so it is a case of being in the right place at the right time.
Try this place in Christchurch. It is a great place to visit and staff are helpful
http://www.yellow.co.nz/site/thewatergarden/
Another Christchurch shop, where the staff are keen plant enthusiasts, is Organism.