Thanks for this thread, I have also been looking for a way to humanely dispatch 3 turkeys in a few weeks time and having done cockerels in the past by neck dislocation I don't feel I am strong enough to do this properly with even bigger and stronger birds.
I was thinking of an air pistol / rifle and a shot through the back of the head into a straw bale, but am liking the look of that small captive bolt kit, it seems cheaper to buy than an airgun, too.