I haven't changed them enough to know if the gasket normally comes with a thermostat or not but was surprised when it didn't. I replaced the thermostat on my 1995 Camry 2.2L with a new radiator. The fit was ok and it seems to work (radiator hoses got hot) but after buying everything off Amazon I had to make a hassle trip to the auto parts store to get the gasket.
Mishimoto MMRT-CAWBK Coolant Reservoir Tank - Aluminum, Black Compatible
9 Review
π§ Improved Stant 10230 Radiator Cap for Effective Engine Cooling
9 Review
Mishimoto MMRAD-WRA-87 Performance Aluminum Radiator for Jeep Wrangler YJ/TJ 1997-2006: Enhance Cooling Efficiency and Performance
9 Review
π₯ Enhance Performance with Derale 13950 Atomic-Cool Remote Cooler, Black
9 Review