If so, where do I get one?
You cannot get one in-game, as it is not part of the Unova dex in Black/White. (Even though some non-Unova Pokémon are available in the post-game, Eevee is not one of them.)
However, it is possible to transfer an Eevee from a Gen 4 game (Diamond, Pearl, Platinum, HeartGold, SoulSilver) using Poké Transfer. Here is a step-by-step guide to help you.
Please note that Poké Transfer is only available after you beat the Elite 4 and Team Plasma.