I can remember from my previous playthrough that I got the Incinium Z from Kukui in Malie Garden but this time I didn't get it from him. is there something I've been doing wrong or is my memory wrong and I got it later on?
Malie Garden (from Professor Kukui if the player started with Litten) Iki Town (from Hala after defeating him in a rematch if the player did not choose Litten)
I'm guessing that you didn't choose Litten as your starter so you'll have to go to Hala
Source