60% O-rings
Various durometers and colors.
FYI, on durometers:
The hardness of rubber compounds is measured by the Shore A durometer; the higher the durometer, the harder the compound.
Typically 60% keyboards ship with an o-ring in the 55-60 range.
Featured Product: JTK Kframe Keycap Display Frame
This is one of the more interesting finds I have come across recently. Anyone who has stocks of keycaps waiting for a home can find one for about $10.