Hack
You cannot breed Dittos therefore you need to find one in the wild with 6IVs. That's pretty much impossible.
You have a 1in 1073741824 to get one. (Shiny Odds are nothing compared)
The only option you have is to hack. Powersave, PkHex are common means. Powersave is relatively simple as all you need to do is click a few buttons on your computer and you're done.
Trade
Reddit has a thread for 6IV dittos but its hard to find a time when its open - BreedingDittos
There are many other sites you can use such as Smogon and NB.
If you don't like hacked Pokemon. Then have fun with the odds because I'm pretty sure all 6IV Dittos have been modified or hacked