I went there one time and I had no idea that there were Smeargle there. Now I found that out and I really want one. But I totally frogot how to get there.
There is a secret entrance to a small part of the Ruins of Alph in Union Cave which leads to a patch of grass full of Natu and Smeargle.
http://bulbapedia.bulbagarden.net/wiki/Ruins_of_Alph
You need to go through Union Cave, which is below Violet City.
Here is a video to help you get there.