The chances of finding a Ditto are pretty poor in Pokémon GO. It's one of the rarest Pokémon. It’s not something that’s possible to farm, given the very long odds of the ditto spawn rate. Just keep playing and you'll find it eventually.
The situation that makes catching a Ditto hard is, that it appears in the wild as disguised itself. It can only be revealed once the Pokémon have been caught. So, keep searching in the wild. It's not possible to find it's encounter rate since it appears in the wild randomly. You can also participate in Ditto-related feild and special research missions.