KI Locations:
Snail Stop: Buy a key item for 9900 G
Dorino Cafe, 600 AD: Talk with Toma to get a key item
Dorino Inn, 1000 AD: Talk with the worker, and then his wife to get a key item
Zenan Bridge: Talk with the knight captain, and then the cook in Guardia Castle in 600 AD. Wait until "Pipe down! What do you know?" pops up, then try heading out of the castle to initiate this. Grants one key item..
Denadoro Mountains: Holds two key items, one immediately after the two bosses, another at Tata's house once the bosses are beaten. Remember that Masa&Mune's spot is a bit harder than normal for most early game parties, but is still usually manageable.
Sunken Desert: Holds one key item, and is one of the first "end game" dungeons to be accessible to most parties. This, and all other similar dungeons with key items, have been toned down to be beatable by low level teams, but levelling up a bit is still recommended. Levels 10 and above should find this doable. You need Robo to claim your reward for this.
Frog's Burrow: With the Hero's Medal, go here to open the left chest for a key item.
Giant's Claw: Holds a key item, and is accessible by pouring Toma's Pop on his gravestone in 1000 AD. The claw itself only exists in 600 AD.
Guardia Castle, 1000 AD: Once you have handed over the Prismshard, head here with Marle in your party. Gives you one key item currently. Also notable as the place leading to 2300 AD if you have the pendant.
Arris Dome: Gives a key item. Is the easiest check of all in 2300 AD.
Geno Dome: Gives a key item. Does not require Robo, unlike vanilla.
Sun Palace: Gives a key item. The boss here typically has the best stats of all the end game dungeons of 600, 1000, and 2300 AD.