Where do you live? I know I've seen it at Latin American and Asian grocery stores, which are sprinkled all around the city. I'm almost positive you could find it at the big Chinese grocery store near the Tetuan metro stop (go out the "General Margallo" exit and keep going straight--it's the first store on the left). There are two or three Chinese grocers on the first two blocks of this little street, but the first one on the left has the biggest selection. If you are in the center, there's a tiny little pan-asian grocery on calle San Bernardo between the Gran Via and Santo Domingo. It's in an arcade on the west side of the street. And Lavapies has tons of little stores where I'm sure you could find it.