You can get thermostats from Repco, Autobarn, Supercheap, Bursons etc etc...
When the thermostat is stuck open, the coolant cools down before it heats up
so the coolant running past your temp sender will still be relatively cool. The idea is that when the thermostat is closed, it aids in warming up your engine. As the coolant reaches a certain temperature, the termostat starts to open to allow the coolant to pass through the radiator, thereby maintaining an optimum temperature.
It's all good!