Frogadier is available at the Friend Safari with it's Hidden Ability, Protean.
The Friend Safari is in Kiloude City, which, unfortunately, is only accessible post-game, after beating the Elite Four.
Another downside, you need to have a friend registered in the PSS, and that friend needs to have a Water Safari, that contains Frogadier. Confusing, right? Basically, it's very hard to get a Protean Frogadier unless you find a person whose friend code creates a Water Friend Safari containing Frogadier.
A more easier way of doing it is to trade through the GTS. Now you can leave a message when asking for Pokemon on the GTS, you can just say 'Hidden Ability Plz' or something. Though, not always trustworthy, it's probably the better of the two.n :D
Hope I helped. :)