We have our son on probiotics. They actually prescribed Culturelle, but if you are going to swallow something, it is better to get a combination with more than one strand. Lactobacillus is one of dozens of bacteria 'normal' people have in the gut, but thanks to CF most don't survive the antibiotics.
What we use, I got at a local herb shop, they had a whole fridge full of differnt options, and I just asked the lady running the place. She recommended one they had with 10 strains that was targeted for kids. I paid $19.99 for a months supply (30 pills).
FYI, we just started about 6 weeks ago and were seeing some improvements, until we did 2 weeks of Timentin & Tobra IV, now we get to start over again <img src="i/expressions/face-icon-small-disgusted.gif" border="0">