Anyone knows where I can get Dittos?

Well you need the National dex first. After you get it head to the Mansion on Route 212. Save your game, then go inside and talk to the owner of the Mansion. Sooner or later he'll say something about seeing Ditto in his garden. Just turn the game on and off again till he says hes seen a Ditto, the head out back to his garden.

You can Breed Ditto with any Pokemon that has the ability to breed unless, the other Pokemon is also a Ditto.

It cannot breed legendaries tho it can only breed Pokemon such as starters, wild, and gift Pokemon, so keep that in mind

Get the National Pokedex from Prof. Rowan after you complete the Pokedex and talk to him, then go to Route 212. Go to the Pokemon Mansion and go in Mr. Backlot's office and save the game. Talk to him. He should say that a Pokemon is in his garden and then him and his butler will have a big fuss about whether he is lying or not. If he did not say Ditto, restart your game and do the same until he does. When he does, go into the Trophy Garden and catch one. Also, you cannot use Ditto to breed EVERY Pokemon, as you cannot breed any legendary Pokemon except for Manaphy to get Phione, and you cannot breed a male Pokemon, as the Ditto MUST be male for the process to work with a female Pokemon. The other Pokemon must be a female. Any starter or evolution will work even if it is a male, I tried it with an Infernape and Empoleon. You can also breed with a male and female of the same type of Pokemon. Dunsparce also works. You can catch it when you have the national Pokedex and talk to Dawn, then her sister. She will say there are Pokemon in any place. If she does not say Dunsparce, save and restart, but change your time to 23:59 and in one minute she will say a different Pokemon. If she still does not say your wanted Pokemon, repeat the process. You can also find out about the Pokemon on the news by examining a TV.