They can be found on TV after you beat the Elite Four the first time. After you defeat them, and end up in your room, go downstairs. Your mom will lead you to the TV and show you them on TV. Then you choose red for Latias, or blue for Latios.
Latios' appearances can be found here.
And Latias' here.
When they will appear next time though, I don't think that is ours to know.