You can get an Oval Stone in Pokémon X and Y in the Unknown Dungeon, on a rock to the left of Mewtwo (hidden).
For help getting the Oval Stone click on this video here.
As for getting multiple Oval Stones, the only other way you can an additional Oval Stone in your game without trading one over is by playing on the PokéMileage Club, Ballon Popping attraction on Level 2.
You can find out more about this here.
Hope I helped :)