If you are in the US, most feed stores and Tractor Supply have it under the name of Corid or Amprollium. In Canada it is called Amprol and may have to be bought at the vet's. In the UK it is Coxoid. It comes in powder or liquid. Here is a link...
Sorry for your loss. Can you treat your chickens for coccidiosis with Corid 1 tsp powder or 2 tsp liquid for 5 days? Your sick chickens sounds like she had it. Any new chickens brought onto your property can get coccidiosis since there are 9 different strains...