How can I safely support an 18 inch Granite (http://tanbrown.org) overhang?

Hi,
I need help. I am installing a lot of Granite (http://tanbrown.org) . On one place it is 129 inches by 24 inches. There will be a 18 inch overhang. Ithis is a straight bar piece. I was told I need 4 or 5 corbels that are 8 to 10 inches deep for the support. I have two children, so I am concerned if that is enough support. Also where can I find corbels that will have the support? I am only having luck finding decorative corbels. I want to make sure that I have enough support for this 3cm Granite (http://tanbrown.org) . Thanks to all who can help me with thier advice.