Go to Pewter City then talk to this guy that's wearing orange and black at the PokeCenter. He will say that there is a swarm of _____ and a certain place. He also says that the swarm changes every 10 minutes. After that, a different swarm comes in on a different route. In the new update, if a new swarm appears, then the server itself notifies of the new swarm in chat.
Example Text: There is a swarm of *pokemon* on *location name*
*Klink is only obtainable in swarms, however its evolution, Klang, can be found inside the Mausoleum of Origins.