Dynamite can be found in the overworld or in some caves. Notably in the Hanging Cave, you can find a bunch of dynamite in a crate next to the Modern Axe. Once you have dynamite you're going to want to look for some small ponds. These are much smaller than the various lakes or other bodies of water. It's hard to see how many fish are in the pond, so you'll need to test the waters and throw dynamite in and hope you get enough fish. Saving first is recommended, that way you don't lose a dynamite. Also, saving and reloading will allow you to try the same pond multiple times until you get lucky enough to get 6 or more fish.