A call to the breeder would help a lot. Ask if any of their remaining chicks from the same hatch as these chicks have had the same symptoms and have been dying. That will tell us the source of the problem is with the breeder.
From my own experience, when a chick suddenly goes lethargic, followed quickly by lameness, then death, it's due to petroleum distillate exposure. Petroleum distillates are in glue, paint, fuels, insecticides, paint thinner, etc. Anything with a strong odor of benzene will contain this powerful deadly toxin.
Another source of toxin that produces the same symptoms and death is mold. Any mold is dangerous. Check the feed, bedding, water, any fruit or veggie treats for signs of mold.