The Ninja Guild doesn't exist here (I literally checked, it isn't there). But the place to buy any supplies/gears for your Ninja is located toward the bottom left.
You need to go down from Warp Girl in Amatsu City, then take a left toward the bottom, then go down again. You will notice a building with a red mask and a few fans besides it. That's where all your supplies can be bought. The building right next door to it is where you can buy gears.
