Call the number given (in the link I posted) for your state lab and ask. It is different in every state. My lab used to be free, but now charges $45.
You can take in a culled bird.
I'm sorry that you've been having so much trouble.
I would send a bird to your state lab for a necropsy. Make sure to put the carcass in the fridge for storage, not the freezer.
Here is a list of labs and contact info by state.
Thread '🔴 State and NAHLN Laboratory List 🔴 [UNDER CONSTRUCTION]'...