I imagine you didn't get an answer because this is a complex issue with a lot of different opinions, and there certainly isn't a single best answer. Most chicken resp illnesses are viral and incurable; even when one is not lethal, they can remain carriers, sometimes get sick over and over, etc., even when treated with an antibiotic. If mine ever got one, I would try to identify it (county extension, ask vet about local resources, local ag college, etc.) and quite possibly cull the whole flock. And most people's chickens never get one. I've read several comments on here along the lines of, I've kept chickens for 40 years and never seen one. IMO, the best way to avoid it is biosecurity, which for me certainly includes never going near a chicken auction, chicken show, etc.
The only meds I've ever bought for chickens are BluKote, and stuff for worming or to control mites/lice. Otherwise, I use my own first aid supplies once in a while, like Neosporin, peroxide, and Betadine. And yes, I've kept chickens for years.
There aren't many meds for chickens out there. Often people use meds intended for cows or other livestock. I worm with Ivomec (ivomectrin,) a pour-on cattle wormer, bought at
TSC. You can buy poultry dust at our feed store for lice/mites, but I usually use Adams flea and tick spray or dust, which of course is intended for dogs and cats. Penicillin G, Tylan and Baytril are probably the best antibiotics to keep, from what I've read, if you are going to do that. I've never shopped for them but imagine they are not sold as chicken meds, either. And most antibiotics are expensive and have a fairly short shelf life.
Somewhere on here there is a very long list of supplies, if you want to search for it.