Where to get them
Since probiotics are a plant-based fiber, eating a diet heavy in fruits and vegetables is a good start. They're plentiful in common produce you're probably already eating. The bad news is that they're at their most beneficial when eaten raw. That's fine for things like bananas (delicious!), but a little rougher for garlic and asparagus.
If you can't choke down raw asparagus-and-garlic salads, slightly steaming your produce should retain most of the benefits. Or opt for more onions in your diet, which have prebiotic benefits both cooked and raw -- just make sure you stock up on breath mints, too. Other foods rich in prebiotics include Jerusalem artichokes (aka sunchokes), leeks, and chicory root. None of those float your boat? Surely you can find something you enjoy in this more complete list from Monash University: