Below is a list of all the costumes for Eight in Dragon Quest XI S. Eight is one of two characters that has a joke costume during a story event, which you can’t change out of during that event. This doesn’t stay after the story event is over, so you don’t have to worry about it.
The first costume you will likely find for Eight is during the middle portion of the game. Once you get to Gondolia, find the two merchant brothers and haggle with them, until they reach 5,000G. If you happen to miss this during that time, then you can simply progress the game until you reach the final part, where Eight will come equipped with it, so you won’t permanently miss it.
Both pieces of equipment needed for this costume come from the recipe, General Directives, which can be found in the Fortress of Fear in the second act. If you happen to miss it there, then it can be found on the First Forest Whale Way Station in the final part of the game. In order to forge both pieces of equipment for this costume, you will need the following items: Technicolour Dreamcloth (x2), Evencloth (x5), Pitch Pearl (x2), Finessence, Dragon Hide (x2), Gold Nuglet, and a Saint’s Ashes.
The Field Marshal
Both pieces of equipment needed for this costume come from a recipe. That recipe, called Field Manual, is found behind one of the Ultimate Key doors in the Cryptic Crypt dungeon in the final part of the game. In order to forge both pieces of equipment, you will need the following items: Serpent Skin (x4), Wing of Bat (x2), Night Stick (x2), Finessence (x2), Gold Nuglet (x2), Sainted Soma, and Grubby Bandage (x2).
The Unswerving Companion
This costume is exclusive to the Definitive Edition version of the game. In the final part of the game, you can head back to Heliodor Castle to do a little unnamed sidequest. You will have to fight a boss at the end of this event, which is outlined on the An Old Friend page. After finishing that event, you will get the Twin Eagle Armour and this costume.
The Warrior King
In order to get this costume, you will need two pieces of equipment, Drustan’s Helm and Drustan’s Armour. Both of them are found in areas involved with Drustan’s Trials. The Helm is found in the Disciple’s Trial - Eerie Valley area, while the Armour is found in the Luminary’s Trial - Citadel of Spite.