I would check a real plumbing supply house. It does make a difference sometimes. If you installed the tank to bowl bolts wrong it will leak also. If the bolts are not tight enough it will leak. I would double check to make sure that is the problem before you go and get anything else.
I would toss that toilet in the trash. It is old and should be upgraded to a nice new 1.6 gallon. The money you spend will pay for its self from water saved.