I buy mine at GNC. I like to mix them up between the GNC chewables and AMerican Health (pink/white bottle). Both kinds are to be refrigerated after opening.
I take them by the handful, but not within 90 minutes of my anitbiotics. As was explained to me, you can't overdose on them. They're really just dried yogurt cultures (I think). It's not uncommon for me to take over a dozen per day.