A List of ESPERS and where they are Found:

Alexander:
You can find Alexander on the throne in Doma Castle after you defeat Wrexsoul

Bahamut:
Beat the Terror in the skies Doom Gaze.

Bismark:
In Magi-tek research Facility.

Carbunk:
Same as above.

Crusade:
Beat all 8 legendary Dragons.

Fenrir:
After beating Phunbaba in Mobliz.

Golem:
Get him in Jidoor auction house after you escape Vector.

Ifrit and Shiva:
Beat them both in the Magi-Teck Factory.

Kirin:
At the top the tower in Zozo.

Maduin:
Magitek Research Facility.

Odin:
In the Ancient castle.

Palidor:
On the beach on solitary island after getting the airship.

Phantom:
Magitek research Facility.

Phoenix:
After you locate Locke in the Phoenix cave.

Ragnorok:
Inside the weapon shop in Narshe in World of Ruin keep it as an Esper.

Raiden:
Find Queens statue in the ancient castle.

Ramuh:
At the top of the tower in Zozo.

Shoat:
Magitek reaserch Facility.

Siren:
Top of tower in Zozo.

Seraphim:
You must pay a theif 3,000(world of Balance) and 30 in world of Ruin in Tzen.

Starlet:
In Owzers bookcase.

Stray:
Top of Tower in Zozo.

Terroto:
In the bone statue in Umaros cave in World of Ruin.

Tritoch:
On top of the cliff in Narsh in World in Ruin. You must fight to get it.

Unicorn:
Magitek Reaserch Facility.

Zone Seek:
Purchase at action in Jidoor for 10,000.