Where is Entei?

Entei can be found in the Dark Cave in the Johto region only after defeating the Elite 4.  You need to find Eusine and he will give you a quest to get coal by defeating monsters in the Dark Cave and lighting the torches.
 

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