The feed stores will carry heated dog bowls too, and they can come in different sizes. The one problem is having birds walk into one, and get frozen feet, or those roosters with big wattles. The muffs on EEs and SFs haven't been an issue here.
Last winter we had one cockerel, our first bird ever, have frozen toes. I don't know what he did, but it was ugly.
We have the dog water dish at the barn, for our barn cat, and any chickens who make the trip there in winter.
And having safe electrical outlets at the coop will be so worth it for you!
Mary