At present you can't get the Hidden Abilities for Grookie, Scorbunny, and Sobble.
As is the norm for a new generation, some of the Hidden Abilities are not available through normal play, and must be received through event distribution. This is particularly true of the local region starters. Once they have been distributed, however, those who have them will likely start breeding them to get competitive tier IVs, with correct Nature, and Egg Moves where applicable. This generally creates breedjects, some of which will have the Hidden Ability, which most people "dispose" of through Wonder Trade (Surprise Trade? Not sure why it was renamed...).
