Where is Suicune?

Suicune can be found via a quest line after defeating the Elite 4 in the Johto region.

If you accidentally defeat defeat Suicune, he will respawn in 7 days. Once you capture Suicune, he will no longer respawn.

The quest line starts in Cianwood from Eusine.
Walk North to South in Path (Suicune Part I)
Legendary Suicune (Suicune Part II) – Route 42
Legendary Suicune (Suicune Part III) – Dragon’s Den
Legendary Suicune (Suicune Part IV) – Mt. Mortar: Middle Entrance – Waterfall Up