Jesuit missionaries from Portugal will appear in Japan in the middle part of the Sengoku Jidai period.
The Portuguese will arrive on the island of Tanegashima in the year 1543. From there they will eventually spread to other ports. This spread is random, but the odds are stacked in favor of ports who are near to the ports where they are already present.
If you have access to a port, where the Portuguese have arrived, then you can meet with the Portuguese. You can purchase arquebuses from the Portuguese.
Christians can purchase arquebuses at discount prices, but they cannot recruit Yamabushi and Sohei. Towns with a temple may riot, if their Daimyo converts to Christianity.
Dutch traders will appear in Japan in the late part of the Sengoku Jidai period. The Dutch will arrive at Usuki on the island of Kyushu in the year 1600. The Dutch bring increased revenue from trade at ports.