I use the Planet Bike Superflash, and while it is water resistant, it is not water proof. If I think I'll being encountering wet conditions, I cut a rectangular piece of something like Saran wrap and place it over the light. I place a rubber band around the mounting base of the light and the wrapping film. I use the same trick on the control head of my bike computer. So far, so good. The Superflash on the seat post is most prone to intrusion by water. The one on the seat stay does not seem to get as wet.

This may work as an interim measure until you can find a waterproof light.