I know you get at the end of a forest like thing. But I found one when I accidently wandered into the grass (at the end of the forest like thing).
They are not very rare in the Pokemon village. They have a 50% chance of appearing in purple flower patches. They have a 75% chance of appearing in yellow flower patches. Ditto can also appear in a Friend Safari along with Chansey, Eevee, and Smeargle Sources: Source 1 Source 2 Hope I helped!
In the purple flowers, it is a 50% chance of seeing a ditto. In the yellow flowers, it is a 75% chance of seeing a ditto. So not really too rare. Also , Ditto can't be found anywhere else (from Pokemon village) , except friend safaris, where it is a 25% chance. Unlikely, but still not rare. Ditto wouldn't be too difficult to get in XY.
[Source][1] [1]: http://pokemondb.net/location/kalos-pokemon-village