Does anybody know where to find Yveltal in Pokemon Y version?
Yveltal can be found in
which you will eventually visit.
Yveltal is not missable and must be caught in order to progress.
Because XerneasX/YveltalY must be captured in order to progress the plot, the 31st Box in the Pokémon Storage System will not become available until then. This is so that they are caught even if at that point, the player has a full party, reached the normal limit of 30 Boxes, and filled each one with Pokémon.