Hi, those caps are extremely hard to find. The Korg EX-800 module uses the same switch+bezel+cap combo. I've been looking for many years also for an alternative and or equivalent replacements. You may be lucky in finding some tactile switches/cap but the height and dimensions will differ and your RK will not look original. I've been able to find new switches, I have those if you need them. Those switches are made up of 3 parts, the switch + a bezel that snaps onto the switch and the cap that snaps into the bezel.
Hope this helps you a little