For what they are, specialist dish heaters are very expensive. However in the past I made one that worked, but with no thermostat. All you need is a metal box (I used one that biscuits (cookies) came in around Christmas time. I cut a hole in the side and fitted a light socket to take a 15watt bulb. The cable to a power point was made using 3 core cable, two to the base of the light socket and the 3rd screwed to the side of the box as an earth return.
It did before I was able to afford a proper heater.
One other alternative I have considered in the past was a heated rubber mat such as those used by physiotherapists to apply heat to a patients back. They have a variable heat setting and are a lot cheaper than conventional dish heaters