Gekokujo is a mod for Mount and Blade Warband set in the Sengoku period of Japan. The name is a term often translated as "the low-ranked taking over from the high" and was chosen because it was a running theme during that era: Oda Nobunaga descended from a clan of deputies to the nominal governor of their province, but the Oda usurped them and toppled their neighbors; Toyotomi Hideyoshi began his career as a cook and sandal bearer and ended up the ruler of Japan; Tokugawa Ieyasu was an upstart who probably fabricated a claim of descent from the Minamoto clan--the only ones allowed to be granted the title of Shogun. As a player of Gekokujo, you will start from nothing and attempt to gain control of the entire country, making the title fitting.
How to adquire it?
It's randomly sold in the store, if you're asking about the armor.
Its not sold in normal stores? The nanban armor?
of course it is , in every shop ,