Need Tank Bolt Seal for Calla II Toilet

I recently installed a Porcher Calla II toilet. I discovered that one of the tank bolt seals was split. The bolt seals are not the usual flat washer type. The seals are made of a soft rubber with a stem that surrounds the shaft as it is inserted into the bottom of the tank. This provides the seal and prevents leaking. I have tried to find this seal in several hardware stores and cannot find it. I tried to use the flat washer type seals but these don't appear to work. I contacted Porcher and explained the situation and they just sent me the same flat washer seals that didn't work. Does anyone know of a place where I can get them? I attached photos of the seal that I need.