Final Fantasy Rebirth » Final Fantasy X » Cactuar Village
When you obtain the airship, you'll be able to fly back to Sanubia Desert, and take part in the "Village of the Cactuars" side-quest. In order to initiate this quest, you must go to where the large sandstorm is. On the southern most edge of the sandstorm, is a rock with a cactuar depicted on it. Speaking with it will allow you to begin the quest. It will give you hints as to where each Cactuar is, that you have to find. When you find each one, you will start a small minigame, where you have to touch the cactuar. If you win, you receive a sphere with the Cactuar's name on it. If you lose, you gain a Sphere del Perdedor. Once you have obtained a sphere, return to the rock, and place it in it. When you do so, the rock will give you a hint as to where the next cactuar is. However, we have all the Cactuar locations here for you, so you don't have to guess!:

Cactuar # Location
Cactuar #1 Oasis - The small pond of water you first saw upon reaching Sanubia Desert
Cactuar #2 Sanubia Desert - East; Above the tent with the Save Sphere
Cactuar #3 Sanubia Desert - West; By the sign near the Cactuar Rock
Cactuar #4 Sanubia Desert - Central; You should spot two of them running around the ruins in the west
Cactuar #5 Sanubia Desert - East; By the Save Point, under the tent
Cactuar #6 Sanubia Desert - Central; In a treasure chest, in the west
Cactuar #7 Sanubia Desert - West; Exit, and enter again. It's in one of the sand whirlpools
Cactuar #8 Oasis; Go onboard the airship, and you'll return to the Oasis. Step away from the Save Sphere to start the event, then re-board the airship, and to the outside deck.
Cactuar #9 Sanubia Desert; Go back to the Cactuar rock, examine it, and it will appear behind you.

When all nine spheres are in place, the sandstorm will vanish. Inside, are two treasure chests. One contains the Mercury Sigil, which is needed to fully activate Rikku's Godhand, and the other item is dependent on how many named spheres you obtained (NOT the Sphere del Perdedor):

Named Spheres Prize
0-2 Spheres Potion
3-5 Spheres Elixir
6-7 Spheres Megalixir
8-9 Spheres Friend Sphere