Z-Crystals distribution:

MeleMele: Normal trial, Bug trial, Rock trial
Akala: Grass trial, Water trial, Fire trial
Ula Ula: Electric trial, Ice trial, Ground trial
Poni: Flying trial, Fighting trial, Ghost trial

Dark Crystal from Gladion after final battle with him
Poison Crystal retrieved from Guzma
Psychic Crystal from Kukui after capturing Solgaleo or Lunala
Steel Crystal in the post game from Colress in Aether Paradise
Fairy Crystal in the post game from Champion Diantha after beating her in battle
Dragon Crystal in the post game from Zinnia after successfully obtaining Zygarde 100% form