Heartwood and (if you feel suicidal), Sanctuary.
However, most of my good ones have come from either the boss fights, or doing the multiple-part quest (over and over) that requires the map from killing a priest of mondain, 20 gaman horns, and 3 minotaur artifacts from the Labyrinth.
Of course, one of the side benefits of doing certain fletcher and carpenter quests is that you can rarely get runics for those skills (without having to fill BODs)