That's all that's necessary as far as location. There are a few US states that prohibit the sale of antibiotics for animals without a vet's prescription. Oregon may be one of them. This may mean that your local feed store cannot sell over-the-counter antibiotics for your chickens. But try...
It's possible this isn't a sore but a squamous cancer. That's possibly why it won't heal. I had a hen with one on her back, and it was extremely painful for her. I euthanized her.
It might be a very serious staph infection requiring an oral antibiotic. You can try treating it with amoxicillin...