I don't know if you have a Southern States near you,but I found it in the cattle section. It's cheaper and probably better to buy the liquid,you get more out of it than the powder.
I recently had one of my chicks pass away from coccidiosis. Her symptoms were like your chicks, and I would give her electrolytes or Gatorade. If she doesn't drink it on her own, I would drip a little onto her beak with your finger. See should drink it from that. If/When you get Corrid,you will...