Titanium Nitride bits are adequate for many applications, but double-
ended Cobalt bits are a much better investment.

This site sells them 12 for $5.00:
http://www.hobbytool.com/browseprodu...rill-Bits.html

I have a friend who bought them much cheaper at a mobile tool sale,
but he can't remember which seller it was. He bought about 2 dozen
for about $5.