Fiji is AMAZING, and Taveuni was my favorite. Unfortunately I didn't get a chance to spear while I was there...but it is on my list of places to get back to and try! The diving is fantastic. Where are you planning to stay? I'll throw my vote in for Paradise, Taveuni. The owners (Teri and Allen) are AMAZING! They were in the process of redoing it when I was there, and I know that they have turned it into a fancier place, but I am sure it is well worth it (although I am not sure if they would be open to spearfishing since they do cater to the scuba diving crowd...although to do offer fishing charters so perhaps they'd do a spearing trip too). At least worth inquiring...I'd go back in a heartbeat!
http://www.paradiseinfiji.com/