Toto uses two suppliers for their flush valves. If the one you have is made by Korky (it has a removeable top, the other one has a plastic screw on the top), you can pick up a seal at any plumbing supply, most hardware stores, and Lowes for about $2. It takes about 30-seconds to replace once you've done it the first time. There may be some crud caught underneath the sealing surface, but once you have it out, it's often easier to just replace it rather than try to clean it. the instructions are on the package.
If you have excessive water pressure, things like this will leak when they start to age. You might also want to pick up a pressure gauge, screw it onto some place like a washing machine supply, the drain on the WH, a utility sink, or maybe an outside faucet (if they aren't shut off for the winter). Leave it for 24-hours and make sure you get one with a second, tattle-tale hand to record peak pressure. If the static or peak pressure is greater than 80psi, you need a PRV and an expansion tank. That will help the life of all valves in the house.