Here is a link for the sulfa antibiotic:
http://www.allivet.com/mobile/p-8021-bird-sulfa-smz-tablets.aspx?sku=50881-2&gclid=cjwkcajwokdxbraaeiwa4xnqv70dhsylu-brqvmcuarjojz_exj4she1ikwoxoaydfm0aznml8bllhoclj0qavd_bwe
I would try to get a diagnosis either by testing, or through a necropsy. Since this seems to be spreading fast, I would get them separated. A necropsy is a fast and easier way to get a diagnosis. Contact your state vet to ask about that or testing. You can spend a lot of money on medicines over...
It sounds like there is a serious respiratory disease going through your flock, such as infectious coryza or others. Is there a bad odor around the faces? I would separate the sick chickens from the others, and either put them down, or get some sulfa antibiotics from a vet to treat for possible...