I spent a long, hot summer as a student engineer working for Procter and Gamble in Kansas City, Kansas. The plant made Zest soap among other things. The tallow used to make the Zest came into the plant by rail in huge black tank cars. The tank cars had coils of pipe inside so one could hook up a steam hose to the heating coil, turn on the steam, wait a few hours, and quickly drain the warm, melted tallow out of the tank car. Filling and draining tank cars is rather messy, so there was always some tallow that splashed onto the cars and on the ground. Add days of breathless summertime heat and intense sun in the city's Missouri River bottom, and the tallow quickly went rancid, leaving me with an unmistakably, unforgettably pungent memory of those days.
And, no, to this day I cannot stand to use Zest.
:roll:
Anyways, back to your question.
Look for a thermostatically controlled pail/bucket heater. There are heaters that are just a band that fastens around the pail and others that add a layer of insulation for faster heating and energy efficiency. Be sure to get one specifically rated for plastic (polyethylene) containers. The ones not rated for use with plastic can get hot enough to melt plastic -- not good for the bucket, nor for your oils. There are some heaters that are not thermostatically controlled -- they just have a high temperature safety shutoff -- and I don't think I'd want them for soaping fats, even though these heaters are cheaper. I also would not get an immersion heater -- a wand that you stick into the product. They are messy, hard to clean, and because they are mostly used for heating water, they can get very hot and have no thermostatic control.
A nice one, but $$$:
http://www.amazon.com/dp/B004HL3VAO/?tag=skimlinks_replacement-20 A reasonable one, less $:
http://www.amazon.com/dp/B0095ZDKB2/?tag=skimlinks_replacement-20 The less expensive one can be wrapped with insulation for faster heating. If you get an uninsulated band-type heater, put the heater near the bottom of the pail. Warmed fat is less dense than cold fat, so it will want to rise. That means you'll get faster results with the heating band low on the pail. You can also improvise a pail heater with a heating pad (the kind you use for a sore back or whatever) and some common sense.
Maybe you've seen all this already ... but if not, I hope this helps!