Every state has a different charge for necropsies, and some like California are free. Tractor Supply usually does not sell vaccinated birds due to the extra cost. Here is a thread with instructions on shipping a bird for necropsy...
It is hard to tell sometimes without knowing more details. Do they free range near places where they could find a decaying animal or fish carcass, or near composting vegetation? Have you wormed them, and with what? Did they have a Mareks vaccine? Any mites or lice on the skin? Coccidiosis...