I too think this is a good medication because few bacteria are resistant to it, and think that a probiotic is needed. Like Rue said, human probiotics can be substituted when you can't find ones made for animals. Just make sure they are not dairy-based, as rabbits cannot digest the sugars (lactose and others) in cow milk well. You can take just about any human probiotic and give that. If you can find a chewable table he'll take and eat on his own, great. If not, you can get capsules and cut them open and sprinkle on wet veggies, although 2 of my guys won't eat veggies that have probiotics sprinkled on them. Another way is to take a tablet and crust it up, or cut open a capsule, and suspend that powder in water or pedialyte and force him to take it via oral syringe. The dosing for probiotics is not exact--I like to say 0.5-1g for a medium sized bunny daily, as needed. I never weigh it out though. I know several other Canadian members have had a hard time getting veterinary probiotics before so it may just be best to go to the human pharmacy.
