I've found these to be working ok, and there are different versions available.
But myself I build one of a regular cheap digital thermostat and it is overriding a speedometer when given temp is reached.
-If you want to offer a cheaper product in your store I don't know other ways than find a good product and resell it or construct your own out of cheaply/readily available parts that you buy in large quanta and try to manufacture it yourselves somehow. (But beware of the heavy legal involvement in developing products for the electrical market as you need to guarantee the fire safety and stuff - which can be tricky when you have a product consisting of parts from several manufacturers in any part of the world! :surprised
Good luck, whatever you do!
Ken
