You can get the harpoon gun by buying the DLC for the map as mentioned above, you also get new vehicles and more weapons with it. Or you could either play co-op with a friend who has it or play on their console with you playing on your profile while their profile is logged in as player 2 - this'll get you the DLC temporarily and will become unavailable when either the co-op game ends or when the DLC owners profile is signed out. Cheap way of getting dlc and achievements.
Also:
jamakinmeorny:we asked where the *** to get it not simply how fucking good it is we know its sick , and who the *** plays halo3? ur a *** bro get ur *** wet fuckin homo, ur possible the biggest homo ive ever met, next time answer the fuckin question dont try to make some homo ass strip tease
Why the hell did you register just to post this comment? What the hell is up with you posting that?
No-one cares about what you think, get over your pathetic excuse for an existence and stop trolling. STFU and GTFO.