I have taken a couple classes on animatronics, and we learned about moulding, but you will need to find molding supplies, and they usually come in a 2 part mix. You would mould your keycap than pour it into the mould.
You would also need to carve or make the cap that will later be moulded, and the easiest way to get detail would probably be with either wood carving or starting with a piece of clay and sculpting that. Sorry that this isn't any product info, but instead the basics on how you would do it.