Is the plastic bushing still inside the "cup" of the seat? Who's the manufacturer of your seat assembly... Garelick? Take a look.
Possible places to get it...
-Garelick (or whatever manufacturer)
-A Grady dealer - likely the easiest - you can spend huge amounts of time researching this, or just give them a quick phone call and maybe pay a dollar or two more than somewhere else.
-Any marine dealer
-Amazon (measure your seat post size. Is it fluted or tapered?)