Prayers for all the trials you are facing.
As a couple of folks have already suggested, I wonder about some sort of toxin they are getting into. I think if the word poison was used in speaking with the vet, she likely thought deliberate poison. I am thinking something not deliberate: weed or insect killer on thnigs they eat, contaminated water--not their supplied water source, but maybe some water that is in some don't of cart basin or puddles or ponds? Also, how do you store your feed, and is there any chance that that is the problem? Damp or wet feed can mold, and the mold can be extremely toxic. Putting new bags of feed into a container or feed dish that has the mold can spread it to the new feed.
Specifically ask the state vet to do a necropsy. With this many birds lost to no obvious cause, they need to find out what is going on before it expands beyond your borders. If they want you tp pay for the cost, express the concern that you have what appears to be an epidemic, not an isolated case. Hopefully that will get sufficient attention to get you some help. Each time you speak with them, make sure to ask when you can expect to hear results. Also, ask for suggestions in caring for and any preventative treatment for the remaining birds.
I agree with the suggestion to separate the birds who show any symtoms of illness from those who appear to be healthy.
In response to a few earlier posts suggesting West Nile as a possibility, it is one of the few diseases that chikens are immune to. That said, there are a number of mosquito-borne illness that chickens are NOT immune to, but chances are pretty good that these would be affecting other bird owners as well.