You can get it from wild Pokemon.
There is a 3/65,536 chance that Pokerus will be on a wild Pokemon. If your Pokemon attacks a wild Pokemon with Pokerus, the Pokemon that attacked will receive the virus. Capturing the Pokemon works also, but running away from the Pokemon without attacking it does not.
So you already have an insanely low chance of finding a Pokemon with the virus, and since there is no indication that a Pokemon has the virus in-battle, you are more than likely to simply run away from the Pokemon. So it is highly unlikely you'll actually ever find it.
If you actually get a Pokemon with the virus, the virus will spread and other members of your team may receive the virus too. I recommend keeping a Pokemon with the virus in your PC if you actually get it, as Pokerus disappears quickly, but in the box it will never go away. So you can get the Pokemon out of the box, get the virus to spread and make EV training a little easier at any time.