The following are the times of when you can receive or find the Espers:

Before you receive the Dawn Shard

Belias (in story line)
After you receive the dawn Shard

Adrammelech (side quest)
Cuchulainn (side quest)
Zalera (side quest)
Mateus (in story line)
After Judge Bergan has fallen

Zeromus (side quest)
After Sakikawood Gate is fixed

Exodus (side quest)
After Visiting Archades

Chaos (side quest)
Shemhazai (in story line)
After receiving the Treaty-Blade

Ultima (side quest)
Zodiark (side quest)
Hashmaul (in story line)
Famfrit (in story line)