Start from the index of Canti popolari italiani if you can't put your hands on the actual book that features lyrics, score and background. Of course you have "Bella ciao" (La me nona l'è vecchierella), La Montferrina, Donna Lombarda, Mamma mia dammi cento lire, La mula de Parenzo, La bella polenta, Tu scendi dalle stelle, Gesù bambino(2 Christmas carols). You also have this wiki list.
But do check all are in the public domain.