From Cherrygrove City, you're going to need to head northward to reach Route 30, and ultimately run into the home of the strangely-elusive Mr. Pokemon. Unfortunately for you, however, getting to ...

The north exit from Cherrygrove City is Route 30, which leads to Violet City. Stop in the house here and the man inside will give you a Berry. Berries also grow on the tree outside his house.