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Depending on the model of your toilet you can get the tank at any plumbing supply house that sells Kohler IF the model is still in production. You're probably better off with original Kohler parts but if you made the lock nut on the flush valve up tight (with the gasket inside the tank under the flush valve) and the flapper is seating properly then you shouldn't be getting any leakage from the tank into the bowl. The triangle gasket only makes the connection between tank and bowl tight. It doesn't keep water in the tank.
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