Heatran is relatively easy to find.
You need to go to the scorched slab shortly after you defeat and/or
capture Groudon/Kyogre in ORAS. To get to the scorched slab, go
under the bridge on route 120, the same bridge where Steven gave
you the Devon Scope. Go down the stairs in front of the first
patch of tall grass you see when coming onto route 120, leading out of
Fortree City, and once down the stairs you must surf under the bridge to enter the Scorched Slab. After then, you need to progress
through the cave until you encounter Flannery, who exclaims that she
is looking for a Pokémon that is said to be found within it. Traverse
to the bottom of the cave and you will find a portal. Interact with
the portal and you will get challenged by Heatran. Also, if you
explore the cave, you should find a Charizardite Y.