Crabrawler can be found in Berry Piles on Route 2, 3, 4, 5, 8, 10, 16 and 17. It can also be found in Berry Piles in Ula'ula Beach, Poni Wilds, and Poni Plains.
Route 2, 3, 4, 5, 8, 10, 17, Ula'Ula Beach, Poni Wilds, and Poni Plains all have a 100% encounter rate from Berry Piles when a Pokémon is encountered.
Now, there is a way to guarantee that a Crabrawler appears. If under the Tree is a large berry pile, a Crabrawler will always appear. However, there is only an occasional chance of a Crabrawler in a Small Berry Pile. Look at my Source for more information. So, find a large Berry Pile in one of the locations I listed, and you will get a Crabrawler.