Depending on things that Russ already asked (what equipment in what box, insulation, ...).
One manufacturer that comes to my mind would be lm-therm (located in Germany), see here in English: http://www.lm-therm.de/en/index.php
. Look for "Control cabinet heating" there.
They have a broad range of heating solutions, for wide power supply ranges, with integrated thermo controller, ...).