Dolben leathers advertise on Ebay. If you search for "Skiver" you will find them, they sell .5mm leather in a range of colours approx £9 for a whole goatskin.
Norman, thanks for the tip. I just bought a goatskin skiver from Ebay. Should be big enough for the wide angle belows , but I also want to make two bigger bellows so any info. about suitable cloth wouldbe gratefully received.
Pinholemaster, I plan to use black paper strips inside the bellows.