In the USA, you can get nice leather from:
http://apps.webcreate.com/ecom/catal...goryFullID=205
I have their sample books, and some of the skiver and composite leathers may be thin enough for a bellows. You can also sometimes get nice thin goatskin on ebay, just make sure that you check out the thickness before buying. Most of the garment quality leathers that are thin enough would make a nice bag bellows, but I'm not sure how well they would work for a folded bellows are they are extremely flexible.
Bonded (composite) leather:
http://apps.webcreate.com/ecom/catal...roductID=23842