The method used to date involved hand casting with a 2 part mold using a liquid silicon or polyurethane rubber. The buttons are cast using a resin. Problems have been encountered with bubbles, color matching, and uncured resin. If this route is used again, a pressure chamber should be used to reduce bubbles. Heat curing may be necessary depending on the resin used. Color matching to lenses will be difficult.

More on the how to of molding and casting can be found on Smooth-On's site

Other methods discussed include Injection Molding. It was found to be cost prohibitive. ($7000 for the mold).

From http://www.matrixenterprises.com/molds.htm, RIM molding sounds like it may be a cheaper option but I have not followed up with them to find out more. Firefox let me know if you wish me to do so.

The types of casters involved have included hobbyists and "cottage" casters that are involved in model making (Casting_Fool and SCModelWorks). Given the lack of success with the latter group, I'd try a more industrial group next.