Im sure you could save money by purchasing a new space heater with a built in thermostat opposed to buying a new thermostat for your pre-owned space heater. There are a few on amazon that would heat a small room effectively for about $50 that have a built in thermostat.
http://www.amazon.com/Lasko-751320-C...ef=pd_sim_hg_4
or
http://www.amazon.com/Lasko-755320-C...ef=pd_sim_hg_3