Might try
http://www.mnrelectronics.com/defabu1.html and ask if they have those buttons. If not them I know there's a company that does sell those buttons, just have to search around. Been talked about on a Envoy-Trailblazer forum, same problem. The buttons on the Delco radio in my Trailblazer seem to be made of a white translucent silicone rubber so light can shine through, with a black coating that wears off.