I have found antibiotics at the local feed stores and our Southern States store. I have also seen some of the antibiotics at Tractor Supply. It's best to educate yourself as to what medicine you need for your particular situation - fairly easy to find suggestions on this site - and then go looking for it. I have no idea where to get prednisone but suspect it may require a prescription.
Prednisone will require a prescription. Most antibiotics are "off label" for chickens, some can be used in chickens, cows, swine, horses etc....such as duramycin, aureomycin, tylan and so on. Once you know the name of the product you wish to use, it might be best to ask for the product as used in cattle; aureomycin as an example...then you have your antibiotic for your chickens.
Tylan is another good example, the store personnel know it's used for cattle, but not for chickens...so, you'd need to go to the cattle section to find it.