Komasan S can only be found in the Oni Evolution Update, which can be downloaded for free from the Nintendo eShop.
After completing the main storyline of Fleshy Souls, he can be found at Nom Burger in Downtown Springdale. He can be battled once a day until he's befriended through the regular method. You can get him more after the first time to trade.
Connect Fleshy Souls with Yo-kai Watch Busters, then complete the second story, talk to Wiglin and select the first option to receive the Honke Manju. In the basement, talk to Slitheref to meet Komasan S. You can battle him once a day (make sure your Yo-kai are level 30 or higher) until he's befriended through the regular method.
When the game is connected with Fleshy Souls, the player will receive a Fleshy Furoshiki from the Post Office. That item can be used to battle Komasan S, where he is located next to the train station in San Fantastico. He can be battled once a day until he is befriended through the regular method.