It is no longer sold at feed stores since 2017 when the FDA had most antibiotics removed. This was done to prevent some from using antibiotics and contributing to antibiotic resistance. But many vets will not see chickens, so the ones who cansee a vet are usually the ones who might be treated. Sometimes you can get around it by using injectable oxytetracycline in refrigerated medicines at feed stores (needles and syringes are needed as well to remove it from the vial.) Tylan or tylosin was a good alternative, but it too is no longer available, unless the pigeon supply companies get it back instock. You might try getting doxycycline for pigeons online. It will treat MG/CRD symptoms, but won’t cure it. What symptoms are you seeing in your sick bird?