I just went though this because I blew a head gasket. But I believe it was caused by the O-ring seal on the pipe that runs from behind the thermostat housing. First you need to replace both O-rings for both of the pipes that run to the heater core. Next I would do a compression test to make sure you don't have more wrong than just a bad thermostate if this is the problem.

But if everything is ok on the compression test, you can get a thermostat at NAPA that is the exact match of the one that comes in the car. Next if you haven't got your gaskets I would recommend getting a set (of intake) from BAP auto parts or from Rock Gasket's part # IG353. They are thicker and better made that the others I have seen. The thing is I clipped the O-ring and had to do it twice, so go rent a pressure tester and plug off the hoses coming off the steel tube going to the heater core and make sure the O-ring is in stalled correctly. Or you might find yourself doing it again.