There's a number of places selling A/Bs for aquatics, prices have gone nuts in the past couple of years along with everything else - but I've no doubt, .gov will find a way to block them eventually. Be careful on Amazon, a LOT of stuff pops up in searches but almost 1/2 of it is herbal stuff.
I need to expand out some of the A/B lines I'm still missing and plan to do that soon.
Pills will need to be crushed, watered and injected (using a large-ish needle) into something the animal likes to eat. Grapes, for example, are beloved by both chickens and ducks. It's almost not possible to get that fluid into a syringe and out a fine needle so you'll need to know if the A/B needs to be intramuscular or ingested, although some intramuscular do work when ingested. Putting something in duck water is not a good idea as they rinse eyes and nares with it - plus splash it all the heck all over the place, so they won't be getting the dose you want. Chickens are more controllable. I've been lucky so far, while I have the supplies, I haven't yet had to stick any of my kids, and I dread the day.