Get rid of the chlorine in the tank; that's why you're here looking for parts.
That seal becomes angled and the tower suctions to the seal, causing lots of problems to flush, sometimes people snapping off the trip lever.
It is a 210/211 flush valve seal and it's a common repair part everywhere.
Go to mansfields website if that's not possible locally.
Make sure when you replace it that the seal free spins in the grooves, otherwise you'll be back stating it's leaking.