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 “20% off”

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.