Any rubber washers designed for use in a toilet should be okay. If they leak, two things: first, sometimes it helps if you lightly sand any roughness off of the casting before installing (only takes a few seconds), and second, you may not have the washer in the right place. Inside the tank, there is no washer, just the rubber and the bolt head. Washer on the outside and the nut. If the tank is not cinched down so it is (almost) touching the bowl, it could leak while flexing.