It is a shame your firm doesn't treat chickens, but luckily, they did a fecal float test. You probably already realize that coccidia is everywhere in the world, likely several different species. That means that a fecal float will always present oocysts whether that is the problem or not unless the chickens are in an indoor sterile environment.
Your chickens look old enough to me that they have long become resistant to the species of coccidia present on your property.
I'll wager, the problem isn't coccidiosis. The initial Corid treatment would have already resolved the issue by this time in almost all cases.
Are you seeing blood in the stool?
I will further wager the problem is bacterial, not protozoan or helminthic.
I once had serious diarrhea in one of my flocks. I thought about worming them but I decided to have a fecal read instead. Hoping to discover what type of worms were causing the problem. I tried about 25 vets around me. Most wouldn't do it even though any vet CAN do a fecal for worms, they just wouldn't for a chicken. I found a large animal vet 40 miles away in Illinois who agreed to do so. He didn't treat chickens normally but as a farm vet he saw more than dogs, cats and parrots.
The chickens had no worms but a severe case of clostridial bacteria that tetracycline fixed.
In all the fecals that you did, what kind of worms did they show?
By the way, there really isn't anything wrong with getting chickens from TSC.
NPIP isn't a magic bullet. It only means the flock has tested Pullorum/Typhoid clean. All the hatcheries that supply Tractor Supply are NPIP as well.
An NPIP breeder's flocks could just as easily be harboring any of dozens of bacteria or viruses.
