The only way to get Zorua is through an event that passed.
Yes, to complete the Pokédex, you need Zorua and Zoroark.
According to Reddit.com,
"They are part of the unova pokedex meaning if you cant get them in game then its impossible to complete the pokedex."
According to Psypoke.com:
"you need both of them to complete the National Pokedex."
That said, here are some ways to obtain them:
According to Gamesradar+.com:
"Zoroark is obtainable in B&W via an event in Lostlorn Forest if you've obtained any of the shiny legendary beasts from the GameStop/Wi-Fi download event. Zorua is also obtainable via an event in Castelia City if you have the Celebi from the 2011 US mall tour or the B&W pre-launch GameStop event. Don't worry too much though if you miss out on the events – Zorua and Zoroark aren't legendaries, so you should have no problem finding a kind person to breed one for you and trade."
If you can't obtain them this way, then try out diehardgamefan's suggestions. (http://diehardgamefan.com/2011/03/08/pokemon-black-and-white-obtaining-zorua-and-zoroark/)
I hope this help Sqealbert!