valve in Villeroy and Boch toilet

I have a part in a Villeroy and Boch toilet that has worn out, and I don't even know what to call it, let alone where to find a new one. It is a small black rubber valve in the fill assembly. The shape is a sort of circular accordian shape, like the fingertip of a rubber glove pushed in and out again.

What is that valve called and where can I find one?