Those herb stores are called "herbolarios" and they are great for all sorts of things--spices, herbal remedies, organic grains, dried flowers, raw honey, teas, health food, etc.

The people who run them are often skilled herbalists who can "prescribe" herbal remedies as well.

There are loads of herbolarios in Madrid (and I'm sure in the rest of Spain). My favorite is at Duque de Alba, 15--right around the corner from the La Latina Metro stop and near Plaza Mayor.