The counties of Hawai'i, Maui (islands of Maui, Moloka'i, and Lana'i), Honolulu (island of O'ahu), and Kaua'i have licensed dispensaries.  So, every island except for Ni'ihau, Moloka'i, and Lana'i will most-likely have a dispensary in the near (within 5 years) future.  

Hawai'i's medical marijuana law is quite stifling - when compared to certain other laws - and the bureaucratic process has been very slow; so the dispensary process is slow.  We don't know which cities just yet, but I'm assuming that you'll see them near urban centers (like downtown Honolulu).