Call the Hunt/Bienfang/Elmers (formerly the Seal Brand) tech line at 1-888-240-6021 They are located in Statesville, NC.
They should be able to help you solve your problem and tell you where you can buy a new thermostat. The thermostats for the 500T and 360T are still available. Getting parts should not be a problem. I would think your machine has some additional years left in it after you put in a new thermostat. The mechanical thermostats last anywhere from 6 months to 6 years. Their life expectancy is quite erratic.
Check the nuts, bolts and sleeves in the hinging mechanism at the same time as this is the main mechanical problem that occurs with the 500T. If they are getting worn it would pay you to repair them at the same time.
Alan