Maractus Pokémon GO is a difficult cactus to catch. It's one of the few region exclusive Pokémon, and you can only normally find Maractus in Central and South America. If you're not in this region, you'll have to go without until an event comes along.
If you somehow do find yourself in Central America, South America or the Caribbean, you can find Maractus easily anywhere in the region.
Maractus is featured in an upcoming Egg update. Unfortunately, you still won't be able to hatch a Maractus if you're not in the proper region.
Maractus Pokémon GO: How to Catch
Some events like GO Fest will let you catch Pokémon that are outside of your region. It's just a waiting game for such an event, but there's a way to trick the game to let you catch Pokémon outside of your region.
If you're feeling brave, you can use a VPN with a GPS spoofing app to make the game believe you're in a different part of the world. The set up is a little different between devices, but you'll almost always have to shell out some money for the VPN service at least.
Maractus itself is a generic pure Grass type Pokémon. It's not worth using in battle, but you'll want to catch one when you can to fill out your collection.