There are three Gourd Seeds you can find the entire Ashina Outskirts area.
Past the sculptor's idol, you'll find General Naomori Kawarada. Defeat him to get a Gourd Seed, along with a Prayer Bead.
After defeating the Chained Ogre, you'll have to go inside a fortress wall. A Gourd Seed is found on the floor, in the corner room to the left side of the wall. It's hard to miss.
Sold by Battlefield Memorial Mob for 1000 sen in the other side of the main building in the Ashina Castle Gate Fortress. (Accessible only after defeating Gyoubu Oniwa). From the idol location, take the main stairs leading to the building then another stairs on the side of the building to reach a campfire. This is where you can find this merchant.