Pokémon specific Z-Crystals
(I'm assuming you've completed the game so you should already have Decidium, Incinium , and Primarium Z)
Show a lady in a Steelix house in Seafolk Village an Alolan Raichu
Complete Kagetoru's quest in Royal Avenue (video tutorial)
------Eevee User Locations------
● Vaporeon User Polly: Third floor of the Trainers' School on Route 1
● Flareon User Chad: Lobby of the Tide Song Hotel in Heahea City
● Jolteon User Jane: Malie City Community Center, in the sitting area.
● Espeon User Ishaan: Geothermal Power Plant on Blush Mountain
● Umbreon User Braiden: Hau'oli Cemetery
● Glaceon User Rea: Iki Town, in the southwest house.
● Leafeon User Linnea: North end of Hano Beach, near the Hano Grand Resort
● Sylveon User Kira: Seafolk Village, in the Steelix ship in the east.
● Eevee User Kagetora: Thrifty Megamart in Royal Avenue (this is also where you start the side quest)
Marshadium Z (unobtainable)
It was only distributed through an event ("Mythical Marshadow") from October 9th, 2017 - February 23rd, 2018. Unfortunately, it is now unobtainable (in Sun/Moon). When the player collected Marshadow, they were given Marshadium Z at the same time. Also, a held Marshadium Z is automatically removed if the Pokémon is traded between players.
Mewnium Z (unobtainable)
A Mewnium Z was available to players who accessed Pokémon Bank from January 24th - October 2nd, 2017. However it is now unobtainable.
In Konikoni City, a woman and her three Pikachu can be found in the north of the city at Lighthouse Point. When the player first speaks to her, she gives the player Pikanium Z. Afterwards, if the player brings her a Pikachu, she can teach it the move Volt Tackle, which Pikachu requires in order to use Pikanium Z and the Z-Move Catastropika.
Pikashunium Z (unobtainable)
Like Marshadium, It was only distributed through events. Now it is also unobtainable. When the player collected Pikachu, they are given a Pikashunium Z at the same time.
Snorlium Z (unobtainable)
Bundled with a Snorlium Z Munchlax event. When the player collected Munchlax, they are given a Snorlium Z at the same time.
Ruins of Conflict (after catching or defeating Tapu Koko)
Aloraichium Z Location
Eevee User Locations
Mt. Tensei Marshadow