Any body from normal diablo and up can drop it. Ive gambled a bonesnap in act 2 (nm or hell can't remember) on 1 of my single player classic barbs. But its just a random chance, on another classic barb i ran normal diablo, NM andy, NM meph, NM diablo, hell andy for about 3 weeks and never found sigons or bonesnap. Got on a hammerdin and found sigons in about 10 runs.
Hammerdin needed irathas and searched for 2 weeks and found them on a barb in about 2 days.
Good luck and stay strong!!