Toilet?

If you are talking about the seal between the water header tank and the toilet pan it is called a 'doughnut seal'
Easily available from most DIY or plumber supplies
Does not need to be bedded in with any sealant, just make sure you tighten up the wing nut clamps evenly
HOW TO:-
Turn off the water and disconnect the water connection. Flush the toilet to get rid of the water left in the closit
Undue the two wing nuts under the rear of the toilet pan . (these clamp the pan and the water closit together)
Remove any screws holding the water closit to the wall.
Carefully lift the water closit up and away from the pan
The doughnut seal can be clearly seen under the closit.
Just clean everything up, replace the seal and reassemble

Geoff in Wales
August 2009