Wood -> Walk around Old Forest
Stone -> Foothills (I am also pretty sure you can buy stone from the guards blocking you from getting into Kor'Minos at the start)
Metal -> Frostwood (After you beat Alrune (yeah i prob got her name wrong but whatever) go to the next area above. You can find metal from left over areas I think but the way i find the easiest is getting the gifts from the Frostwood Kitsune that you run across)