You can find it when you "headbutt" certain trees. Here's a list of the trees where you can find it.
safari zone. around the middle i think
safari zone on cianwood
In the Pal Park forest.
you have to wait until you get all the way to the safari zone
which is after you beat all the gyms in johto and half of kanto.
it is on routes 26,27,29,30,31,32,34,35,36,37,38,39,43,4… & 48,new bark town,cherrygrove city,violet city,national park,ecruteak city,lake of rage,safari zone gate(headbutt)