It's easy. Simply make a Ditto, lv. 100, set all IVs to 31, make it shiny and from a foreign region (ex. Japan), and you're done... It doesn't really matter whether it is legit or not if you want to use it yourself and not trade either...
For legit Pokemon. you'll need to set a proper met place/level/etc.
Just export your save with a save manager, open it with PKHeX, make the Ditto, and set it in the box as many times as you want, then export the save and import it back to the 3DS.