I always have these on hand for my chickens:
Saline - I have a sterile bag from the vet but you can make your own, too
Iodine
Plain neosporin
Blue kote
Gloves
Syringes/needles and fish tank tubing
Gauze
Vetericyn spray
Vetericyn antimicrobial eye wash
Nutri-drench
Corid
Electrolytes and probiotics
These I went overboard and bought - antibiotics, not sure if you can get them anymore without prescriptions:
Aquamox
Aquazole
LA-200
Tylan 50
Animax ointment (leftover from another animal infection)
I’m a little overkill with my chicken first aid kit. Especially because antibiotics expire, I probably shouldn’t have bought them. But it does make me feel good to have on hand.