Does anyone know where I can purchase some cone shaped rubber washers
for fitting on the long thread that retains the hinge brackets of a WC
Seat?

I spent a fortune on an upmarket WC seat and no matter how many times
I tighten the wing nuts underneath they come loose and the seat keeps
moving out of alignment. The previous seat had conical rubber washers
underneath that self aligned in the holes in the pan. Wish I had kept
them.