1) Burned Tower, part of story
2)Cianwood City, north part, you will battle him.
3) Route 42, so east and Surf, then use Cut.
4) Vermillion City, after beating Elite Four
5) Route 14, east of Feushia City and South of Lavander Town
6)Celurean Cape, where you can battle Suicune*
*You must have finished Power Plant mission and taken Misty back to her gym first.
Hope this helps!