Keycaps from bottle capsLODGE, a Japanese makerspace offers workshops where you can make your own keycaps from bottle caps.
In Tokyo, for about $10, you can bring your own PET bottle caps and recycle them into keycaps by injection molding.
Yahoo! Japan's LODGE must be a pretty cool place. At least that's what I'd call an open collaboration hub where laymen can make molds from scratch and household injection molding machines lay around.
The process is quite easy: bottle caps are available on the spot in various colors but you can bring your own caps too. As a start, the material has to be crushed into 5mm pieces. Then comes the injection molding part with the INAPI machine. Finally, the end product has to be cut and cleaned up.
Knowing the many crazy Japanese soda flavors, the material and color choice won't be limited. In addition, you can use quite any common plastic materials that don't generate toxic gas when melted, e.g. PS, PP, ABS, POM, PA6, PE, TPE.
The mold is DSA profile and the finished keycaps are MX compatible.
Finally, I found the protagonist for you, an INARI manual injection molding machine by Originalmind Inc.