There are total 9 Cactuar that can be found all around the world. The location of the Cactuar are:

Oasis

Near the first Save point

Sanubia Desert (East)

In the alcove to the east, north of the tent with the save point

Sanubia Desert (West)

Search the sign near the Cactuar rock that says

Sanubia Desert (Central)

Near the ruins to the west, look for two Cactuars running around the desert.

Sanubia Desert (East)

Near the save point under the tent.

Sanubia Desert (West)

Trapped in the treasure chest in the far west.

Sanubia Desert (Central)

Inside one of the sand whirlpools. You must exit and re-enter the area.

Oasis

Teleport back to the airship and go on the outside deck.

Sanubia Desert

Return to the Cactuar statue and the last one automatically appears behind you