I am aware that a lot of things will eat them!
The reason I find it an unusual question is because they aren't mass-produced for food to my knowledge. They are difficult to contain, have an complicated life cycle, and are very carniverous as soon as they begin to grow legs. Especially bullfrogs, which don't think twice about eating the bullfrog beside them if hungry.
They are easy enough to catch at night walking the edge of a pond in the summer with a flashlight and net (or gig if you'll be feeding them soon), and there is no shortage of bullfrogs. Many species of tree frog and leopard frog are much more infrequent, and have periodic die offs in bad conditions. Also be certain not to feed anything (but hognosed snakes) toads, as every American species has toxic secretory glands. Several species can cause severe poisoning and even death in humans.
I tend to think that there have to be cheaper, safer, and easier to obtain protien sources than amphibians. If they are in your backyard or you come across them like High Times, that's great. But I wouldn't pay the cost biological supply sources ask. They probably catch them just like HT!