Delta Emerald (mewtwo)

I just beat the E4, now where do I find mewtwo x & y?

You find him in the Pokemon Village, which you can get to through the Winding Woods south of Snowbelle. Be sure to bring a Pokemon that can use Strength, Surf and Waterfall. Possibly Cut too; I can't remember.

Hello, Delta Emerald is not an official title. If it is a hack you are referring to, you should search for its corresponding thread in the ROM Hacking section of PokéCommunity: http://www.pokecommunity.com/forumdisplay.php?f=184

If you actually meant Omega Ruby or Alpha Sapphire, you should head over to their section and post your question here: http://www.pokecommunity.com/showthread.php?t=343254 or perhaps mew_nani's answer was just what you needed!

Pokémon General is actually only for topics that don't fit anywhere else on our forums :) good luck!