Are you sure it is a removable key cap and not part of the physical switch? You can try to contact an authorized Korg dealer and have them order the part for you. Might only come as the entire switch unit though.
Um ok thanks, i was under the impression its a cap because the same exact switches are available with a different lever. And the manufacturer of those does not know/have the caps or a smooth aluminum switch version.
since you guys seem to know the switch, do you have a part number or manufacturer?