Sadpandamonium

Eevee is still in the building with the Team Rocket leader. There is a "Secret Room" that you will need to go through in order to defeat the boss and obtain Eevee. I believe you go to the bottom most floor of the Casino and to the furthest most room to the right. Along the North side of the wall there is a secret door which should open after you engage it. It is also easy to see along the floor because on tile will be lighter then the others. That will be the door entrance.