I'm sorry that your hen is sick.
What symptoms does she have?
Water soluble antibiotics came off the shelf beginning 2017 and are only available by prescription.
You should be able to find injectable Tylan50 at
most Tractor Supply stores. Depending on your area, they may have it locked up somewhere. Some states may not allow sales of antibiotics either.
Injectable Tylan50 can be given orally or by injection. If you can give it by beak

that would be best - so you avoid injection site necrosis (Tylan50 is administered
at least 2X a day) If you can find it and need dosing info, let us know.