They come from all around you. Other peoples ponds, lakes and streams. Frogs have a lot of tadpoles and it get's very crowded once they've all turned into frogs. Plus toads and tree frogs also lay their eggs in your pond. I have 9 ponds and each one has about 8 to 15 frogs in each. Plus a creek runs through my back yard. So a lot of them go to the pond at the end of my road and the rest go to the river eventually.

Here is a link that might be useful: Gabelmans Gardens blog