*****SPOILERS TO GF LOCATIONS*****
There is no way to get about 40-50% of them for you now until you enter the forth disk where you draw them from the bosses there.
You can still get a few though here are some I can think of:

Brothers: Go to the little cave near Deling City and solve the dungeon puzzle thing there and defeat them.

Doomtrain: Go to the centre of Lunatic Pandora by foot and pick up the shiny thing there, it will be soloman's ring, you will need to collect a few random items to cummon him. Only ones I can think of right now are Malboro Tenticles.

Diablos: Use the Magic Latern that was given to you by Cid when you left Balamb to help the Timber Owls. You will need to defeat Diablos to save before you use it.

Tonberry King: Go to some ruins (forgotten their name) in Centra and kill 20 Tonberries to fight the Tomberry King, defeat him and he will join you.

Bahumat: Go to the Deep Sea Research center at the corner of the world map and fight Bahumat. Only walk towards the light when it is not glowing.

Cactaur: Goto Cactuar Isle in western Esthar and fight the Cactuar visible on the world map.

Edan: Get the rest then get a walkthrough ^^