What if you can't find aphids to feed praying mantis babies?

You can feed a nymph (baby) mantid a variety of small insects besides aphids. Many pet stores carry fruit flies which are a widely used food source for nymphs and small mantids. You may also use pinhead crickets (also found in pet stores) if the mantids are big enough. Ask your local pet stores about their smallest feeder insects and see if your mantids are big enough to eat them.