you get it from the poketch place
first you get all the other stuff
use your pokeapp
use a marking map
in Solaceon Town, but to get marking map go to jubilife and to the app station
First of all, you catch Mespirit with the marking map not Alzef. And second, Cressila is the other Pokemon you catch with the marking map.
The marking map app can be found in Jubilife city from the Pokétch company. Talk to the owner and he will eventually give it to you.
Use the marking map to track her down.
use the diamond etc from down the bottom and just6 drag it to where you want it.
You can get the Marking Map app from the Pokétch Company after defeating the Hearthome Gym leader.
you need the marking map on your poketech and the you go between sangem town and route 219 until he will be down in the marking map and then go into the sea there
Zapdos, moltres, and articuno are only roaming around in Platinum.
On the marking map you can't mark Pokemon but if a Pokemon is roaming it will be represented by a Pokemon face.
You can get it from the PokeTech App place in Jubilife City.