You're welcome.

Righty-O on the fifteen millimeter number.
Duh.

Go to the McMaster-Carr website and shop for your cork material and whatever diameter of punch stock you desire.
Also note that there is likely more than one type of cork sheet available on that site.
They have a great deal of selection and an unfathomable selection of gasket materials (like the cork you seek) and other industrial equipment and supplies.
A true gold-mine for the manufacturing, engineering, and maintenance trades.

Cheers
Tinker