Looker Tickets can be found where there are alot of sparkly stars shining on the ground/on the wall - just shiny glittering. If you want to actually see skip to 9:12 on this Youtube Video and in a second or two, it'll show one of the Looker Tickets (Well a bright sparkly spot!)
Here's the locations.
>When you get to the Looker Bureau, Looker will say he wants you to be his assistant, after hearing of your jaunts with Team Flare and you being the Kalos Champion. However, he wishes to test your detective skills and so sends you out to Lumiose City in search of five special tickets, Looker Tickets. They are located as follows:
Magenta Plaza: In the Pokémon Center
Centrico Plaza: In the Gym
Vernal Avenue: In the herb shop
North Boulevard: In the hotel
North Boulevard: In the Museum