If you do it outside good old sunlight is just fine if you want to grow indoors any LED grow light should be sufficient although tomatoes do like a lot of light so you may need a few of them if you decide to go with indoor growing.

You also want to make sure that as little light as possible gets down into the bucket of water to avoid stuff like algae growth. But that's a simple as wrapping your bucket in some nice cheap tin foil keeps the light out, helps keep the water temperature stable, and even reflect some light back up to your plant so that it can get more