I got mine off of Ebay, but I think Harbor Freight sells them as well. They look like a milling drill bit. It worked a heck of a lot better than the version with a spring loaded centering pin. I broke half a dozen of them before I bought this one. The larger size worked great.
Here is a link. http://www.ebay.com/itm/2PCS-HSS-CO...669010?hash=item3f4a405252:g:n6YAAOSwWnFV8xS5
