They are all over the place, LOL No, seriously, it's true - you can find them all over Ivalice (well, all over the part of Ivalice that you can explore in FF XII....)

Here are the details; obviously, (SPOILER):rolleyes2

-CuChulainn is found in the Garamsythe Waterway, under the Royal City of Rabnastre. In theory, you can access it pretty early in the game, but the way to get to it and actually beating it is pretty tough;

-Zalera is found in the Barheim Passage that you explore also early in the game - however, you won't find it right away, you'll need to come back after clearing a subquest;

-Same comments about Exdeath in the Mosphoran Highwaste, Zeromus in the Stilshrine of Myriam and Ultima in Crystale Grande (btw, it's not for nothing that she's called "Ultima" getting to her and defeating her is HARD): those are all places you'll visit at one point during the story, but you need to complete other stuff before coming back and going after them;

-Then you also have the "secret" summons that are located in places completely optional: those are Adrammelech in the Zertinan Caverns and Chaos in the Necrohol of Nabudis;

-Finally, there's the 13th summon, Zodiac, but you need to get 10 summons and complete a specific mob hunt before it becomes available in the Henne Mines