Welcome to BYC!
Clavamox (amoxicillin trihydrate/clavulanate potassium) is also a very good antibiotic, but it's very expensive and it's one of the few that I can't find without a prescription. One could also use the human version, but getting the correct dose for a chicken is difficult.
-Kathy
Generic injectable Baytril - can be given orally:
http://www.mexi-cali.net/eng/index/item/24/29/enfloxil-25-ml-same-as-baytril
-Kathy
Edited to add: with Baytril you should treat just the sick birds and only if you are sure they have a respiratory infection.
The antibiotic Baytril works very well on most bacterial respiratory infections that poultry get, but it is banned for use in poultry, so one should consider that before buying it.
-Kathy