My personal favorites, if I get to choose, are in this order:
Probios dispersible powder ($9 at our feedstore for 240gm bottle which is plenty). It lasts ages in the fridge, 1/4 teaspoon per bird dosage, the powder can mix with water or little bitty snacks that they'll devour. You can use while medicating with -mycins and -cyclines.
Acidophilus capsules or tablets. One capsule's contents or crushed tablet per bird per day, one per six 1-day old chicks. Again because it's such a small amount, very easy to mix into something quickly eaten. Can be used while medicating with cyclines and mycins.
Plain white yogurt. The benefit is the birds if they love it REALLY love it. The drawback is that it's a teaspoon to tablespoon per adult bird per day. If they don't love it, it's a little harder to mix. But you can get it in the middle of the night. It makes an awesome treat to bribe and tame your birds if they like it. Most will learn to like it - just are offended a little by its stickiness at first. Good for mass-dosing. Another benefit is that it is fortified with vitamin D which helps calcium absorbtion. It also contains some fats and is a food so that it is bioavailable. Another drawback is that it's not able to be used while you medicate with -cyclines or -mycins because the high-dairy content and amount of a serving cause the antibiotics to be ineffective.
You can also buy other live-bacteria probiotic products at the feedstore - like Fastrack (which I believe also ahs a yeast culture for more vitamin B). The key is to look for labeling that includes a number and the letters CFU that denotes that there are colony forming units. THe label can say "byproducts of..." or "Fermentation products", but still must have a CFU level listed to show that there are live bacteria, not just their by products.
All of the above are good products - I just feel that in the long run, Probios is the strongest and ends up to be most economical even over plain yogurt. But sometimes 68 cents fits the budget better than $9.
Incidentally, except in cases of diarrhea, you can use babyfood applesauce in small amounts (no more than 1/2 teaspoon per bird) once a week or so to help clean out their digestive tracts and the pectin 'feeds' the bacteria you're trying to get to colonize. I feed it more often to birds with static crops as it helps clean the digestive tract while feeding bacteria. I learned that trick when I had chronic mechanical statis of the crop in one of my pet parrots. It worked wonders to keep her digestive tract flowing.