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Which one? The the thermostat housing can be bought at just about any parts house. For the reinforcing ring, look at ebay or this link. Thermostat Housing Ring
For what it's worth, unless the housing is really in poor condition, I would get the reinforcing ring. The way the housings are made, it is only a matter of time until the bolts get over tightened and warp the thin metal flange again. This reinforcing ring solves that problem.