Doug,  You might try S. LaRose, tel 910-621-1936 .  They carry a wondrous
assortment of tools and spare parts and may either carry the rubber
bellows 'domes' or know someone who does.  These were readily available
some years ago, then went completely out of production.,

Recently, I saw that they were available again but I cannot find out
where.  You could also call Sal Provenzano (last number I have for him is
212-655-7021) who often has replacement parts for later bird pieces.

Hope you find what you need.

Frank Metzger