The Dubious Disc is a hidden item in the Sewers below the Poke ball Shop in Anthian City. Its effects allow Porygon-2 to Evolve.
To get the Dubious Disc, go to the last room before the exit [SPOILER if just got 4th gym badge] where you would fight Professor Cypress. It should be a room with a Pokemon Computer, once you enter to click on the third locker to the left and squeeze in through the door. Once you enter go to the path to the left leading to the top right corner of the room where you will see a Poke ball, touch it and you will receive the Dubious Disc.