If you could post a picture of the part's in question, it would help.
Off the top of my head, I would suggest Looking at diaphram's in aquarium air pump's. But I would need to see a picture to tell you what size of pump to "canablize".
If the part really is a "balloon" check in a Party type store. They will have all kind's of balloon's to chose from.
By The Way ..... Welcome to FSM.