He is still found in the Ruins of Alph in both HeartGold and SoulSilver; however, he is somewhat rare (with only a 10% chance of appearing) so you may have to explore for quite a while before you find one. To get to the patch of grass, you need to enter via the Union Cave (near Violet City) and head down to the lower level, where there are several doorways leading to blocked-off areas of the Ruins of Alph. One of these contains a patch of grass containing Natu and Smeargle.
Hope this helps! :D
