'Live' food covers a huge variety of items. The obvious, like guppies, goldies, and shrimp are found at many stores--though often they are not in the main part of the busines, but rather a separate tank in the back where you can't seem them.
Less common live options include: mosquito larvae, blood and black worms, rotifers, scuds, green water solutions, snails, vinegar eels, insects, insect larvae (maggots), micro worms, and brine shrimp. Most can be raised at home, but are not found at many fish stores--simply because they are a typically tough to make a profit on. Check Aqua-bid, and you'll see many live food cultures that you can purchase. However--keep in mind that while live food is needed for some species, or for certain stages of some species, it is not needed for all fish. Most fish will be just fine with a balanced flake food.
Vegetables are another feeding option.