Fortunately, this is relatively easy to do. There is a product that has been available in pet stores for years. Originally marketed for birds, it has been repackaged for reptiles. Unfortunately, it does not seem to work well in herbivorous reptiles. Acidophilus is found in the refrigerator section of health food stores.
Yogurts containing live cultures are generally available wherever yogurt is sold.
By law, such yogurts have to have a certain amount of live cultures in them and must state their presence on the packaging. In addition, they may display the logo of a national yogurt association certifying their bacillus content. Only a small amount of the acidophilus or yogurt is needed. For larger reptiles, 1-2 cc's once a day for one or two days. For herbivores, you can help the remaining gut bacteria a second way, by giving them something easy to digest.