You can find it at most feed stores, tractor supply, etc. It comes in powder or liquid, either is fine, look in the cattle/livestock section. Dosage chart is attached below, use the severe outbreak dosing. Also give your very sick bird some orally in addition to the treatment below, dosing for that his here: https://www.backyardchickens.com/threads/corid-oral-drench-instructions.1211991/