If you exchange friend codes with other people, you can get different-typed Friend Safaris!
The only way to get a Ditto Friend Safari I guess is to ask around the internet. There is a way you can reset your 3DS console to change the friend code. You can look here on how to reset the console.
Note that it is a lot easier to just ask around, rather than continuously resetting the 3DS console for a Ditto safari. xD
Hope I helped. :)