Anyone know where you can get these floating electric detectors used in the floods?? or how to make one

Have you tried youtube?

My guess would be a screwdriver power tester wired permanently on with an extra power source jabbed through a piece Styrofoam.

You might want to add some kind of audio alert, so it getting pretty hands on. You could always check a basic circuit boarding help sites.

If your in the water, you'll be dead before you hear the alarm lol. So my advice would be just to keep a testing pen/screwdriver handy.