In the US, you probably can only get metronidazole online from petsores (ie-Chewy) or fish supply companies without a prescription. Look for FishZole. In Petco and other pet stores one may find API Natural Cure which has metronidazole in it. Dosage is 250mg per day given orally, for a 5 pound chicken. Allivet, Tractor Supply, and other stores may have the Fish Zole. If you don’t smell a bad odor, it may not be canker. A vet could do a smear and look under a microscope.