I use an aquarium heater. Cheap and works a treat. You can use the temperature controller on them if you trust them or use them in with a separate temperature controller (which I do).
Here's a link to a thread with some discussion about their use. There are others threads also if you want to go digging.
Pic attached of fermenters in small 150L chest freezer with aquarium heater submerged in 8L of water. Freezer unplugged. Heater connected to a Fridgemate. Works a treat. ie. set and leave...