E.A.
If you can't find kefir, use plain yoghurt. You can add it to juice to sweeten it, but I suggest plain becasue you can control how much sugar is in there, unlike the sweetened kind. And a lot of sugar will make diahrrea worse. Give her as much as she will eat without affecting her breastmilk or formula intake, kwim?
You can also buy something called a "probiotic" that comes in capsule form. open the capsule and sprinkle it on her food, or put it in her bottle, if she's taking formula.
Typically, the diahrrea will go away on it's own in time,but the gut flora that was killed off when she was on the antibiotics is important to her immune system, and you might be looking at more infections if you don't give her a supplement to repopulate the bacteria in the gut.
IOW, always always feed them lots of yogurt/kefir/probiotics when they are on antibiotics. If you do, you will most likely avoid the diahrrea.