If you visit any Japanese confectionary store (wagashi shop) you will be able to find Nerikiri :)
Try underground floor of any department stores, such as Mitsukoshi in Ginza, Isetan or Takashimaya in Shinjuku. There is a huge floor dedicated to food and you will be able to find at least 10 different wagashi shops there.
Other chances might be at a tea ceremony. Participate in the longer tea ceremony activities, may be the host will offer you a Nerikiri as a company of matcha tea.