Your go-to supplier for all of the different leathers you will
need to build an organ is Columbia Organ Leathers (CGL):

https://www.columbiaorgan.com/columbia-leather-home/ 

For a small busker organ pump I choose the CGL thin which is
about 0.030".

The CPL in varying thicknesses is great for small pneumatics,
motors, and pouches, and their CPL Valve Leather is perfect for
valve facing and also gaskets:

https://www.columbiaorgan.com/columbia-leather-home/products/leather-skins/ 

The one exception might be for flap valves, where a chrome tanned
leather might be more appropriate because it lays flatter than the
flap valve cowhide sold by Columbia (not an issue if you use springs
or such to pull the flaps tight). That can be obtained from Organ
Supply Industries (OSI) in Erie, Pennsylvania.

Roger Wiegand