Six Great Treasures

Flat Spider 平蜘蛛

 * Talk to Inukai at 7 Trust to initiate the Flat Spider Event.
 * Finish the Flat Spider Event, to do so you have to find 蜘蛛弾正 (Gumo Danjo), who is a random archer grunt, and send Inukai for that battle out and destroy the grunts unit. After the battle, Inukai will kill Gumo Danjo and you will get the Flat Spider(Hiragumo). Note: The event will not trigger if you kill Gumo Danjo in a battle with a higher priority event, such as the final battle to invade a house etc. You will just have to find and kill him again.

Golden Seal 金印

 * Fight wild boars and get a total of 60 "Boar Points"(Boars in Sado count for 20 points, Boars in Owari count for 10. Note that you do not see the number of boar points, but is something you have to keep track of.)

Coke Bottle コーラの瓶

 * Let enemy (Demon Army or Shimazu House) reclaim one province in Africa.

No-shield 盾無し

 * Found by developing Kai's nation power once (The Takeda House must be conquered for this to happen.)

Echizen Crab 越前ガニ

 * Found by developing Sado's nation power once

Giant Pearl 巨大真珠

 * Found by clearing the Pearl Dungeon in Ise twice (requires Sill)

Trivia
The coke bottle is a reference to the 1980 movie "The Gods Must Be Crazy", where a coke bottle is the source of discord amongst a South African tribe. "This is the treasure of the Africa region. It is said that tens of thousands of people fought and died over this in the past".