You're looking for the slide seal or something like that. You'll have to - er, I mean you'll "get to" take the toilet off to do it, and you should probably put on a new base seal for $4 or $5 while you're there.
You might also check how the vinyl hoses inside the toilet are doing while you're there.
The big trick is finding the rear bolt to get the toilet off. You'll find some threads here about it.
Remember what the toilet looks like, because if it's not the original one, there's umpty zillion different kinds.
Don't forget to wash your hands and clean under your fingernails when you get done...