Unlike with Shiny Pokémon, the odds of catching Pokérus can't be boosted; thus, it's simplest to trade for an infected Pokémon and allow the virus to spread to your Pokémon. Pokémon from other countries do not have a higher chance of carrying the virus.
Unfortunately, as the 3DS games have lost internet connectivity, many people are locked out of trading and must find the Pokérus independently. There is no easy way out of this --- you just have to get lucky. Pokérus can spread through the party, but if your Pokémon cure themselves, it can't be transmitted further. You will want to keep a Pokémon with active Pokérus in your PC for this reason.
It's worth noting that you don't have to catch Pokémon to get Pokérus. It has a chance of generating after any battle, so running away is a faster and easier method of rolling the dice.
For further information, including differences between generations, please read the Pokérus article on Bulbapedia.
