professorpaul wrote:
Lowes, Home Depot, or other "big box" store. They have wall mounts, or
kind that go on the heater itself. ...

Unlikely any of those will have temp range below about 50F though, 40F
at outside (which might be adequate but higher than OP asked for)...

Seems like last time this question arose there was at least one at Grainger.

Seems I've seen some in the farm supply online catalogs but don't have
one handy here to look at at the moment; all our waterers have builtin
rheostats so haven't looked for external thermostats...

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