Question: how to get all the zygarde cells?
Answer: Trainer's School: first floor, in the bed right of Meowth (night only) Hau'oli Outskirts: East of the Pokémon Research Lab. ... 
Heahea City: In the back-left corner of the Tide Song Hotel lobby. ... 
B2F: Lab area, at the end of the hallway containing the secret labs.