These guys are the main merchants in the game, and you should run into one eventually without trying too hard. They each appear in a small tent and are often found near a sculptor's idol.
Here are the first three locations you'll likely meet one:
Ashina Outskirts - on top of a bluff, after defeating the game's first mini-boss. Run & Jump from the roof of the building with the gun soldier to reach it.
Ashina Outskirts - near the Ashina Castle gate, after defeating the first main boss, go back away from the gate and up the stairs on the opposite end of the arena and he'll be outside on the right-hand side of a large building with some NPCs inside.
Ashina Castle - right before the entrance to the Abandoned Dungeon, from the first Castle Idol, jump across the ravine to the left and go across the bridge, over the closed gate and past the Bell enemy.
There are several more throughout the game that you can also find later on.