Eevee's hidden ability is Anticipation. Other than by trading or events that have already passed, How can I legitimately acquire an Eevee (or one of its evolutions) with its Hidden Ability?
Besides the one you get from Fennel's assistant in Castelia in Black 2/White 2, you can find Eevee in the Pokémon Dream World. They can be found in the Dream Park, apparently only after you have collected certain amount of Dream Points (I don't have any exact source, but considering its status as a rare Pokémon, I'd say you need 10,000 Dream Points). You have an increased chance of going to the Dream Park when you tuck in a Normal-type Pokémon.
Edit: Source - Bulbapedia
After you beat the Elite Four in Black and White 2, you can receive an Eevee with it's hidden ability from one of Fennel's assistants (Amanita) in Castelia City (She's in the Medal Building).
This Eevee will always be male.
There is a chance to get both a male and female Eevee in all of the Hidden Grottos, but they are a very rare encounter.
- Beat Castelia City's Gym Leader
- From the Pokemon Center, run all the way left and then run upwards to get on the street which the gym is on.
- On the left you should see a building with the image of a coin next to it. Enter.
- Head to the 3rd Floor.
- Talk to the girl on the left with the pigtails (Make sure you have a free slot open for Eevee)
- Now you have an Eevee with anticipation! :)