Catching Pokemon with Hidden abilities
One possibility is a Friend safari. You have to be online at the same time as the friend whose Friend safari you're visiting in order to get hidden abilities from that safari.
>Once the player has been online in-game at the same time as a friend, the Pokémon in that friend's Safari will thereafter have a chance of having their Hidden Ability.
Another option are Horde encounters.
>Pokémon encountered in hordes may also have their Hidden Ability.
Passing down hidden abilities
>In Generation V, female Pokémon with Hidden Abilities have a 60% chance of passing on the Hidden Ability to their offspring when bred with a male Pokémon from the same Egg Group.
>In Generation VI, any Pokémon, including male and genderless Pokémon, can pass down their Hidden Ability when bred with Ditto. Ditto's Ability is irrelevant to inheritance.
As you can see, if the Ditto has the HA, the offspring will not have it.
Hope I helped.
You can't get every Pokemon with a HA from a safari or a horde, as there's a set number of them available. You have to breed all the others.
Dream world is relevant.
>A Pokémon found in the Dream World will always have its Hidden Ability, if it has one. Pokémon found in the Dream World have about a 25% chance of being female regardless of gender distribution, unless the Pokémon can be of only one gender.