I would make my own:http://burningsmell.org/current/
You need a small toroid transformer like this: http://i00.i.aliimg.com/photo/v0/404512784/T130_52_choke_coil_for_LED_Lighting.jpg
You can remove one like this from an old PC powersupply.
Connect the wires so you have 1 winding, mount an LED across the coil, one each way (there will be AC current on the output) so you need the LED's to clamp the voltage both ways.
Connect this to an analog input, and then you can sense when the heater is on.
Another option is to use an opto coupler as shown on the first link.