I would use leather, but if cork is your choice then, as usual,
the McMaster-Carr part number for 12" by 36" sheet is 9487K1 and
cost is $1.68 for 1/16-inch thickness.

Something else they have for the coin piano collector is the lock
for the money box in a Seeburg coin piano, part number 1873A14,
about $30.00.

Don Teach - Shreveport Music Co.
Shreveport, Louisiana