Free: Contests & Raffles.
Don't fall for the "turkey load" advertising. You will end up paying twice as much for the same thing. That being said, any high base between 4-6 will kill turkeys.
I use the Federal Premium magnum turkey loads in a #5 shot. With a Jellyhead choke in my 870 I put 13-14 pellets in the head at 50 yrds.
but I had one close shot that left the shot cup sticking out of the bird's neck just below his head ... that was definitely different.