How to get Zygarde in Ultra Sun?

Question

I went to route 6 after defeating the Pokemon league for the first time and talked to everyone I could (like it said in the Pokemon Database) and no one gave me a 50% Zygarde.

Answer 1

After becoming the Champion of Alola, go to Poni Island and continue the story until you get to Resolution Cave. Zygarde 50% Form should be there and you get to battle it and catch it. Another Zygarde in its 10% Form can be obtained from Dexio in the lab on Route 16. You can use the device in there to merge Zygarde 50% Form, Zygarde 10% Form, and the 40 Zygarde Cells in the Zygarde Cube you get from Dexio to get a Zygarde with the Power Construct Ability. This Zygarde will transform into its 100% Form if it reaches half health to lower in a battle.

Source: The Official Guide to Pokémon Ultra Sun and Pokémon Ultra Moon

Answer 2

go to poni island- the cave at the back of poni meadow. zygarde is inside. Catch it, then go to aether house on rout 16. dexio should be inside. he has the other 50 zygarde cores/cells. battle him for it.
Reference: Experience