My Dying Chickens!!!

I just lost Gloria, my pet chicken. I cried so hard. She's the cutest 1 month-old chicken. I didn't even get to take a picture of her. The rest of the flock are dying. This is not my first time to raise backyard chickens. But this is my first time experiencing this kind of situation. it looks like Mycoplasmosis. Since most of them are coughing, sneezing and have bubbly fluid in the corner of their eyes. Although some of them got well, others just died so quickly. And when I checked on Gloria's body. She had food stuck in her trachea and a tiny stone in her gizzard. What could possibly went wrong. They had food all the time. I changed their water twice everyday. There was no sign of gapeworms. They just suddenly choked to death. I can't afford to lose them all. I had 96 chickens. Now there are only 50 of them left.

Can I still save them or is it too late. 30 of them, i have bought from an agrivet store, where i had bought my chickens before and they were ok. So there's 30 from the agrivet store and the rest(66)i have bought from a man selling broiler chickens at a lower price. Though I can't tell now which of them came from the agrivet and which came from the man.So could it have been the problem? Could it be because they weren't vaccinated yet, unlike the ones we buy from an agrivet store? Or is it just some kind of a disease. Please help...