They do not carry it in feed stores. You have to get online, and the link I posted has it in stock reasonable. It may take a few days to get it. But get started on cleaning the eye, and using the antibiotic ointment right away.
I would go ahead and treat for possible mycoplasma (MG.) Other chickens have probably already been exposed at the same time she was. I would flush her eye twice a day with saline or eye wash. Then apply either Terramycin eye ointment from your feed store, or plain Neosporin triple antibiotic if...