TSC or a farm feed store/CO-OP There is a meds section normally for large mammals. Kind of in the area where you would find Corid.
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I used 22 gauge needles. I would read on the proper dosage to give a bird of their size. Also would read up on where to administer. I just picked the only place I know and that’s the back of the neck. I did read that in the breast area can burn them. But again, I’m no vet and only did this to keep my birds alive. I’ve only had cockerels and roosters that have needed the antibiotics and gave them shy of 1cc. On the smaller cockerel he got 1/2cc.