Spoiler warning: Quest and/or game spoiling details follow.
You can exchange Soul Orbs for Infernal Bolts through the The Sweaty Cyclops Quest. Each orb turns into 3 infernal bolts, with a rare chance to get 6 infernal bolts.
You can obtain 100 Infernal Bolts in The Ultimate Challenges Quest Warlord Level.
Spoiler ends here.