It is available at the dealers.
For goose neck, up to ident. no.: 043 002979, it is A 000 811 01 65 for about $12.
Later one, A 108 811 00 65 for about $25.
Quality is important, I would buy from the dealers.
For the central rear view mirror, the one inside, there is no gasket as a separate part, the mirror came as a whole.