Here are some good antibiotic/medication resources:
http://avianmedicine.net/content/uploads/2013/03/17.pdf
http://avianmedicine.net/content/uploads/2013/03/18.pdf
http://avianmedicine.net/content/uploads/2013/03/09_therapeutic_agents.pdf
This is where it gets tricky... Injectable penicillin is easy, just weigh bird, do a little math and inject in breast muscle. Pills are also pretty easy... weigh bird do math, then cut pills if necessary. Powders are hard, because without a gram scale it's hard to know how much they should get.
Which form? you can get injectable penicillin at most feed stores, but oral penicillin and amoxicillin have to be purchased online as fish antibiotics. Google "penicillin for fish" or "amoxicillin for fish".
Injectable penicillin...
What a great subject! I'm gonna go think about this one, then post some ideas.
Edited to add:
I started this thread - https://www.backyardchickens.com/t/1150128/suggestions-for-a-first-aid-kit-preliminary