depends on location and temps you have to deal with, the base heaters and pet bowls work if you don't get down below -20 for extended periods, if you live up north where we get -30 to -70 a plastic bucket with a stock tank heater works best with nipples, or youcan do like I do rubber feed dishes and change em out more often , all depends on time you have and what you want to do
So a small flock we just had a black bowl from TSC and knocked the ice out in the morning and poured hot water in it. Unless its really cold the chickens keep it open. You about break the regular waterers trying to get the ice out.
Before you buy a heater, please consider whether you are going to use your heater in an area that is shielded from rain, snow and sunlight or if it's going to be exposed to the environment. Sunlight and extreme temperatures can cause the plastic on standard electrical cords to degrade over time. Once the wire is exposed, it's a safety hazard. Likewise, rain can get into the interior of certain heaters and come into contact with electrical components and create the same safety hazards. For example, "Pie Pan" style heaters like the one Sue Gremlin posted above are only intended to be used indoors. They also need to be used with a galvanized waterer, not a plastic one. Check the manufacturers instructions carefully. The last thing you want is to create a safety hazard. Please be safe!