The Aeons: Origins and Significance

Aeons are powerful summoned beings in Final Fantasy X, manifested through the Fayth, individuals who sacrificed their lives to become statues. Summoners undertake pilgrimages to gain control of these aeons, using their immense power to combat Sin. The aeons represent both hope and a tragic cycle, central to Spira's spiritual and physical conflict.

The world of Spira has been tormented for a thousand years by a colossal entity known as "Sin." The teachings of Yevon assert that Sin is a divine retribution for humanity's over-reliance on machina. While summoners have repeatedly defeated Sin, it invariably resurrects, perpetuating an endless cycle of destruction. Tidus, a Blitzball star, is mysteriously transported to Spira after his home city of Zanarkand is obliterated by Sin a millennium in the past. Believing Sin holds the key to his return, Tidus joins the summoner Yuna on her sacred pilgrimage to vanquish Sin and usher in the Calm.

In Final Fantasy X-2, the acquisition of Dresspheres and Garment Grids is tied to various missions and locations, often involving the remnants of past events and the ongoing narrative:

Airship:

Speaking with Brother, Paine, Rikku, and Shinra, then the Barkeep, and resting in the cabin, progresses the story.
Viewing the Treasure Sphere "Journey's Start" is part of the initial events.
Three Al Bhed Primers can be obtained from Brother.
Speaking to Buddy before Brother's broadcast yields another Primer.
Speaking to Brother a second time grants the Festivalist Dressphere (International/HD Only).
Asking Shinra about a Garment Grid Tutorial provides the Vanguard Garment Grid.
Completing the Standard Cup in the Fiend Arena rewards the Psychic Dressphere (International/HD Only).
Completing the Standard Cup in the Fiend Arena three times grants the Intrepid Garment Grid (International/HD Only).
An Ether and other items are found in the Engine Room.

Mt. Gagazet:

The mission "Outrun the Leblanc Syndicate" requires reaching the top of Gagazet Ruins within a time limit. This mission becomes available after returning to the Celsius from Luca.
Reward for completing "Outrun the Leblanc Syndicate!": Black Mage Dressphere, Muscle Belt.
A Mega Phoenix and Elixir can be found on the exterior of the ruins.

Airship (after Mt. Gagazet):

Speaking with Brother and choosing to comfort him is an option.
Speaking to Shinra can provide the Black Mage Dressphere.

Besaid:

The mission to "Find Wakka, and maybe something else..." becomes available after returning to the Celsius from "Outrun the Leblanc Syndicate!".
Objective: Find Wakka.
Reward: White Mage Dressphere, Besaid Sphere, Protection Halo Garment Grid.
To obtain the White Mage dressphere, you must defeat the Flame Dragon.
After defeating the Flame Dragon, head back to Wakka for a scene, receiving the Besaid Sphere and a Protection Halo Garment Grid.