As you reach the main temple area, cross the bridge and you’ll meet up with the Chocobo Knights. After the brief scene with them, continue along to the temple and, in the scene thereafter, speak with Yuna and the temple will open. Cool! Anyhow, outside the temple, you’ll find the shops in the building to the northeast, just next to the temple. There is a chest behind it with four Ability Spheres inside, while a chest in the westernmost part of the area yields 4,000 Gil . Enter the temple when you’re ready.
Inside, there will be a scene with Isaaru before you regain control. Go left of the stairs to the Cloister to find an Ether and east for a Remedy . Then head into the room to the right of the stairs and open the chest for a Mega Phoenix . This is an extremely valuable item: it can revive your party from KO with full HP! Be sure to keep a tight, tight hold of it!
When you are done, head into the Cloister.