I must pass this on to anyone else in need of this service.  Check
out http://www.leatherthinning.co.uk/  I had need of some sheep
leather around 0.7 mm thick but the best skin I could get was variable
between 0.7 and 1.1 mm.  Since the leather is for the organ mechanism
in a large orchestral musical box, it is critical that it is thin and
flexible so as not to put too much strain on the mechanism.

Having cut the strips of leather to size, I sent them off to this
company on Tuesday.  They arrived back on Friday perfectly thinned
to between 0.67 and 0.7 mm which is perfect for the job.  The cost?
Just 1.95 GBP per square foot, which is nothing against the cost of
the original leather and the labour to complete the job.  They have
a special Italian-made machine for the job, and can handle up to 550 mm
wide and thicknesses from 8 mm down to 0.1 mm.

John Farmer, UK
http://www.musicanic.com/