Anima - Baaj Temple
Anima is one of the three optional Aeons that you can acquire in Final Fantasy X, it's also one of the most frustrating and time consuming ones for you to acquire. This Aeon can be found at Baaj Temple, it's the same location you visited at the very start of the game, however, at this time of the game it can only be reached via Airship.
In order to reach Baaj Temple you'll need to speak with Cid and select the search on the Airship menu. Enter the coordinates below and you'll find Baaj Temple.
Baaj Temple (X14, Y57) - You can obtain Anima here as well as Lulu's ultimate weapon the Onion Knight
There's more to do at Baaj Temple than just grab Anima too! When you first arrive you'll fight Geosgaeno who is the big fish guy who chased you into the ruins at the start of the game. He's a really easy boss to beat down at this point in the game and when you kill him you'll be rewarded with a chest that contains Lulu's Celestial Weapon.
This chest is tough to find and the game doesn't point it out for you. Click on my link to Lulu's Celestial Weapon for the exact location of it. Regardless of if you grab the item or not when you're ready to continue swim back towards the green square on the mini map and into the temple.
The temple itself is pretty small and it isn't home to any monsters. Inside, as you can see in my screenshot, there are 6 different statues and 'some' of them will light up for you as you get near them. The thing that makes these statues activate is whether or not you've gotten the bonus loot by clearing the Destruction Sphere parts of the Cloister of Trials.
Bevelle will always be lit up since using the Destruction Sphere is required for completing the Trial. If you've completed all of them up to this point the only one that won't light up is Zanarkand's Cloister of Trials. The reason for this is the first time passing through Zanarkand you're unable to complete the Destruction Sphere part. You will have to return once you have the Airship, aka now.
I won't be writing guides myself for each Cloister of Trials but I will link to the videos I used and that I found helpful in completing this task. Some Cloisters I linked multiple videos for incase over the years some get taken down off Youtube.
Once you've completed all 6 of the Cloister of Trials you'll want to interact with the Glyph in the Antechamber room.