Galangal and Makrut Lime Leaves - These ingredients are the key to giving the soup a unique, authentic flavor. And both are almost always available at any Asian grocery store (look for frozen versions if you can't find fresh). You can substitute ginger for the galangal which produces a still delicious, though slightly different result. You can use the peel of a regular lime in place of the Makrut lime leaves which similarly will still give you a delicious soup, though just not a totally authentic flavor.