Crickets tend to be the most available food, and generally make the basic staple for frogs.
Crickets can, and should, however, be supplemented with special calcium gut pellets, like "Calcium Plus, food for Crickets", that provide the frogs with necessary minerals.
These can be found in most pet stores.
The best way to do this is to drop some of these pellets in with the crickets several hours before they get fed to the frogs.
Then when you go to feed them to the frogs, they will be full of good stuff for healthy froggies!
http://allaboutfrogs.org/info/doctor/sick.html
http://www.nextag.com/frog-food/search-html