Frogs are fished up. They can be found in almost any place to fish in France (and are present in Egypt as well), wherever the fish are jumping. They require no fishing skill, much like an anchovy or minnow. The best bait for them, according to the fishing book, is cherimola blan grapes, which can be purchased, I believe, at the Nectary or the food store in France.
If you are looking for the meal Frog's Legs, you can also buy this at the food store in France.