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.
You can also receive an Entei Egg as an Ultra Rare drop from defeating the Legendary Dogs Boss on Four Island.