I recently just paid an absurdly ridiculous amount of money just for the lid on one of our two Lavac heads, reusing the old seal since it was in good shape. I had to bite the bullet with a $150.00 charge for a round piece of plastic. I remember the lid gasket (seal), to cost about $30.00, plus shipping.
Lavac toilets are incredible, but on the very rare occasion you may need parts, the Brits have us by the nads.
True Blue . . .
sold the Nauticat