I have used a heated dog bowl for just a few birds and a heated metal base with a large metal waterer on top of it for a big flock. A heated plastic hanging waterer just came out last year, but I haven't tried it, yet. There are also directions somewhere for making a heater base out of a metal cookie tin, with a light bulb inside. That's been popular.
No mater what kind you use, it's always safest to keep it elevated off the litter and on something that is fireproof, like concrete blocks and also be very stable, so it isn't tipped over. We've never had a problem.