Where can I purchase a low voltage heater for a bird bath?

If you have a Co-Op or Tractor Supply, they should have one that will work. I got my horses water bucket heaters and water trough heater there.

Got mine from Ace Hardware. Amazon also carries several.

You could try Menards, Fleet Farm.

I ordered from Smith & Foster

eBay

You may want to rethink this. You need to have a "solar sipper," which only has a small opening for birds to drink from. Otherwise, they get too wet and freeze to death later.

I know Tractor Supply has them... at least in their catalog / website. Otherwise a country Feed & Grain store would carry them for use with livestock / horses.