I think if you are wanting Oxytetracycline Hydroxide, you'd probably need to ask your vet for a script or try the fish flakes.
You can, I believe, still purchase LA200 (Liquamycin/Oxytetracycline) at stores like TSC. LA200 is a long-acting antibiotic used in cattle, it's meant to be given by injection once every 3 days.
https://www.backyardchickens.com/threads/miracle-la-200-dosage-and-vetrx-for-crd-treatment.631458/
I'm not sure what the difference between the fish flakes and the injectable would be, many people use LA200 to treat respiratory symptoms, but Oxy can also be used to treat Staph infection unless it's resistant, then you may need to have some testing to find out what antibiotic would be most compatible.